Just checked and while the crossbow takes only one slot, there is no way to select or drag a shield in the other at the moment. Hope it will be added at some point. Would be really nice.
*img
Note that it is also impossible to select the crossbow in the melee weapon slot. I think this will have to be reworked for it to happen cause it seems the game doesn't allow to have two melee sets or two range sets to select from at the moment. And as well, no combination works, dagger + hand crossbow for example.
Extremely disappointing, hope they work on that
