I don't think you can qualify for a second off-hand attack. You certainly can't in PnP.
The extra bonus action (which can be used for a second off-hand attack) is, I think, a homebrew feature of the Thief subclass that I've assumed is to replace something or other in 5e RAW that Larian felt didn't translate well to a computer game, but I confess I've not looked into that further. I'm sure someone will already know
