Maybe it isn't a secret, but I've found that the two-on-one battle is made a lot easier by using a particular exploit. Once you enter the arena and the two doofuses have drawn their swords, run to one of the barrels of water, jump on top of it, then jump onto one of the two wooden fences and position yourself at either of the ends of them.

From there, you can fire arrows at their heads with impunity at point-blank range. Sometimes the Gothic 3 hitboxes kind of shit themselves and they might get a hit on you despite what you're actually seeing, so heal yourself immediately. As killing your opponents is against the rules, just whittle their health down until you think the last speck can be taken care of with a little right-click attack.

When you're ready to finish them off with your sword, make sure that you don't jump off the fence that you were standing on. For some reason, the game detects that as leaving the arena, so just walk off it instead. To make it even easier to finish them off, have them chase you around the fence so you can separate them from each other.

Of course, I did all of this with no offensive magic spells, as I didn't have any at the time and had no idea as to whether or not they're allowed in arena fights. If they are, then I guess all of this is rendered useless.

That's handy! Thanks!

Headknock arrows do not kill, so you can avoid having to get down at all.

I just use the slow motion spell. "time bubble"? (but it is best cast before entering the arena)

Hmm that sounds like an interesting solution... but i usually kite the guys around the arena, works like a charm...