Hello again.
I apologize for how long it took me to get back to you on this one. Unfortunately, after digging into the LeTBS plugin a bit, I can only conclude that I don't know enough about how this plugin works to get it working together with dual wield.
I tried to find the part of the plugin where it processes the attack action specifically, as a starting point for how to make this compatible, but the plugin is massive, and has little documentation. Blindly poking around inside did not prove to be very effective. If support for Attack Times + as a trait were to be built into leTBS, I feel like dual wield would probably work almost 100% with just that change, but since I can't personally figure out where the attack sequence is even processed, let alone what to change to make it duplicate certain parts of it, I will be unable to do this myself.
There are also some unforeseen compatibility issues to do with the lack of working action sequences in leTBS. My plugin relies on secretly adding commands to the action sequences to reset some attack data between skill uses, which is why YEP_BattleEngineCore is a hard requirement. While leTBS supports BattleEngineCore, it does not seem to make use of action sequences at all, which means any compatibility patch between my plugin and leTBS will need to find a new way to reset this attack data.
As it is right now, you can use dual wield and the extensions with this plugin, but they do not split damage, or effects, or hit multiple times. You can still use this to make dual wield actors capable of wielding shields in their offhands, make weapons that can only be wielded in the main or off hands, or two-handed, and make shields that can be wielded with two-handed weapons. As is, the only thing I could suggest to try to get the core features of this plugin to work are to contact someone who has a lot more experience working on the leTBS plugin.
Sorry.