you are corret, thank you very much!
I checked that and if you look closely you can see that sarevok is "holding" two weapons in hand. I changed teh design of my char with Ctrl+6/7 in the Sarevok-Design and found out that that the weapon itself is part of his design, so giving him another weapon (except of his fists) leads to him holding two weapons at once. However, that causes the game to crash if i attack something (also magic-animations since he has none).
I also made him join my party with using Ctrl+Q and really holds his chaosblade in the weapon slot (and in the inventory so he can drop it).
The only problem is, that in order to not crash the game he has to use his fists (which makes the fight not very hard) - i haven't found any weapon that wouldn't bring the game to crash since it always leads to the "two weapons"- thing.
EDIT: In fact there are some 2handed weapons designed for sarevok:
SW2H02SA
SW2H11SA
SW2H20SA
FINSRV
(and some other, you can find them using ShadowKeeper and MatchItemName: Sarevok )