So I'm thinking of going through BG I and II without any mods, starting with a solo class fighter in order to switch to a Kensai at the beginning of BG II and dual-class to a mage later on, probably at level 9. I plan on dual-wielding longswords or katanas. What would be best way to save/spend proficiency points through BG I? I'm thinking overall I'll have about 10, 12 points.

I am pretty sure it doesn't matter how you spend your proficiency slots in BG 1 because you will have to reallocate them when you start your BG 2 game.

I would be careful about picking katana proficiencies because good katanas are very hard to come by (except for celestial fury which you won't be able to get until later in BG 2). However, there are lots of good long swords so if you wanted to dual wield I would put as many points as I could into the dual-wield and longsword proficiencies.

If dual-wielding two long swords seems boring to you like it does to me then you might try putting some proficience into flail when you get to BG 2. You can get the flail of ages very early on by going to the De'Arnise keep which is an excellent weapon and you can upgrade it twice in TOB to make an awesome one handed weapon.

I'm sure there are better builds out there but I really like the flail of ages.

BG1 uses a much simpler system than BG2 for proficiencies, i.e. Large Swords in BG1 includes Longswords, Bastard swords, Scimitars, and Twohanded swords in BG2, so you'll just have to reassign points when you import the character into BG2. (BG1 also doesn't have dualwielding.)

For BG1 take bows and large swords (ranged combat and especially composite long bows are extremely powerful in the game), you'll have to change your proficiencies at the start of BG2 anyway.

Yeah, if I remember correctly, even standard composite bows have a good + to hit, and maybe damage. It's been a long time, but I also seem to remember there being a good one in that first town you go to, down and right from Candlekeep, though it may involved some kind of thievery.