When BG1 character animations are equipped with off-hand weapons, the game crashes due to the following issues:
1. The main creature animations for attacking with two weapons do not exist (A) nor do the main weapon animations (A) or the offhand weapon animations (OA). The most dire issue. Furthermore, you can't simply steal the weapon animations from the 0x5/6### series [i.e. WQL prefix] because you will have to restructure the BAMs to fit the 0x64## framework, namely by either forcing users to use Sprite Mirror, or picking out the Extended [E suffix] series of animations from those BAMs. 2. There is no code to handle equipping a weapon in the offhand. It gets loaded into the main weapon animation vid cells instead. This could theoretically be remedied by putting in some extra code, but to warrant this at all, you would have to remedy #1. 3. The way that the offhand weapon animations are named means that with the current weapon prefixes for the Extended series of animations, the BAM names end up being 9 characters long. Not so bad if you rename all the weapon prefixes for the entire 0x64## series
BG1 character animations can be equipped with off-hand weapons without the game crashing (i.e. fix items #2 and #3 on the list) so that modders can do the missing animation work (i.e. item #1).
Alternatively, you could fix item #1 as well and provide us with dual-wielding capable BG1 character animations ;)