The assertion has been made that the composition of a party has some bearing on the question of how a single class character compares to a multi-class character. The idea being, I suppose, that the sharing of xp alters the comparison between two characters for this comparison.
In terms of total xp gained in the game, the assertion is without merit, since in BG 1 + TotSC, the xp cap is 161,000 xp, *regardless* of how that xp is divided up among party members, the number of party members or just having one party member, i.e soloing.
If you solo a character through BG1/TotSC you will end up with 161,000 xp - if you adventure with a party of 5 companions, you will also end up with 161,000 xp, i.e. each party member will end up with 161,000 xp each.
So total xp available, and how much you, or each party member can amass, is a non-issue, since everybody, ultimately, regardless of party composition, will end up hitting the level cap at 161,000 xp.
Given that the shared xp is the same, regardless of party composition, in any party with a single class Fighter and a multi-class F/MU, the Fighter will, generally, be one level higher of fighter class, compared to the F/M. If the F has enough XP to be 4th lvl, then the F/MU will have enough xp to be 3d lvl as a fighter. So regardless of party composition or the actual xp shared among party members, within that party a F/MU will be a level behind the single class Fighter.
Now the only difference, regarding xp, is the rate at which one acquires it, not the total one can end up with. Having a 6 person party means each will have 1/6th the xp, all things being equal, compared to the single member of a soloing "party". The effect of this is that is simply that the more people you have in a party, the slower they level up since they have to share the xp among their party members.
So, party member(s) *will* affect how quickly one gains levels, although it won't in any way affect how much xp each party member has once they reach the level cap.
Thus the difference between a solo "party", or a multi-member party is simply when each party member reaches the level cap.
Now, given that the game was designed for parties of 6 members and doles out the xp accordingly, the game allows, as designed, for all members of a party to reach the same level cap as any party with fewer members, or a solo character. The only difference in game play is *when* a particular party member reaches the level cap.
So, sure, a soloing character will reach 161,000 xp before the two members of a 2-person party, and before the six members of a 6-person party. But so what? Is it "better" to have your character reach the lvl cap in Chapter 4, compared to a later chapter for multi-member parties? Is it better to top out and reach the lvl cap relatively early, and thus play the entire rest of the game gaining no further levels, no increase in "power" and acquiring no more xp?
I guess that question comes down to play style and what one desires from the game - all well and good, but it has nothing to do with the OP's original question. They didn't ask *when* a single class vs a multi-class will reach the lvl cap, they asked about a comparison of the two alternatives. Since when you ultimately amass those 161,000 wasn't part of the question, it really needn't be part of the answer... :)
So, the comparison. A single class character will, on average, have one additional level compared to a multi-classed character, with regards to the class they share with the single class character. So a 7th lvl F single class will be matched [in terms of total xp] by a 6/X F/X.
So, is this single additional level reason enough to claim that a single class character is "superior" or "better" than a multi-classed character with a level one less than the single class character - *but* with all of the abilities of another class to go along with that single level difference with the single class character?
Our multi-class F/X will have fewer HPs than a single class Fighter - but they will also have means at their disposal, their spells, to mitigate this, by being harder to hit than the single class Fighter. They will have a THAC0 one point worse than the Fighter, but again their spells make up for this - a Fighter can not incapacitate all of their enemies with a 1st lvl Sleep spell, nor kill all of them in one "blow", via a Fireball. Surely, if one can end a battle with a single action, i.e. a spell, then how many HPs you have, and what your AC is, is pretty moot. :)
Same thing with comparing a single class Thief to a multi-class Thief. A Thief will have more HPs than a Th/MU, but fewer than a F/Th - so that's a wash. The Thief will have, on average, 25 more Thief skill points than a Th/X, but will also *not* have the additional abilities of the multi-classed Thief's other class, which can make up some of the difference i.e. a Th/MU's Knock spell, a Th/Cl's Find Traps spell, a Th/MU's Invisibility spell, etc.
In terms of death dealing, a F/Th is a better Backstabber than a single class Thief, because of the F/Th's better THAC0 [using the Fighters THAC0 progression] and better chances to hit and additional damage due to the F/Th's access to weapon specialization, which the Thief doesn't get. All things being equal, the F/Th will hit more often, and do more damage than a Thief.
Bottom line is that since the game nerfed the only meaningful disadvantage multi-classes have vs single classes, i.e Racial class level limitations, there is little reason to play any single class character compared to a multi-class character. The single extra level the single class character can achieve, and the rate at which they level up is, IMHO, completely offset by the multi-classed characters increased abilities due to their having two classes rather than one.
There are really only two exceptions to this; single class spell casters vs multi-classed spell casters having the same class, and of course, classes that can't multi class in any event, such as Monks and Paladins, etc.
Given the way that spell slots are doled out upon level up, a single class spell caster can, at times, have a meaningful advantage over a multi-class character. Whether or not this is an actual advantage depends upon what you want from the character - if you desire the best spell caster possible, then the single class is superior in those terms, while for a more versatile and more "survivable" character, the multi-class is superior, over all.
Much of this depends, ultimately, simply on one's approach to the game. From a power gaming pov, the single class is "better" and will become the "best" quicker than the comparable multi-class. However, this is a very narrow pov if it ignores the other abilities that the multi-classed character gets *from* their other class...
From a role playing perspective, the multi-class character *always* has more versatility due simply to their abilities from that other class.
Personally, I would *always* happily trade away some HP's, a one point THAC0 advantage and advanced weapon specialization that a single class Fighter has, to get all of the abilities of a multi-classed Fighter's other class - be that a F/MU, F/Th, Cl/Rgr or whatever. The fact that my multi-classed character will gain levels a bit slower than the single class is simply not an issue for me, since either character is going to hit the level cap with the same number of xp.
Different strokes for different folks, of course. I would be interested in hearing from someone who disagrees with the above, and why they disagree. For example, in what *meaningful* ways is a F "better" than a F/MU, F/Th or a F/Cl at being a Fighter? In what *meaningful* way is a Thief "better" at being a Thief than a F/Th? etc