Larian Studios Forums Baldur's Gate III Baldur's Gate III - General Pact of Blade and Two Weapon Fighting

Alternatively, you could grab the polearm master feat, which adds a d4 bonus action attack (as well as a few other polearm related things). This attack gets stat bonus added to damage, and since it is a single weapon it will definitely work with pact of the blade. Another advantage is that it definitely gets stat bonus added to damage, whereas generic off hand attacks do not (unless Larian changes the rules).

The downsides are that you must take a feat, and you must wield a weapon with the polearm tag.

If you just want the extra attack, this is a good way to get it. If you actually want to literally dual wield then it won't work.