I'd disagree on that last point, to a certain extent. You could perfectly legally use a weapon in the other hand, same as anyone holding a two handed weapon or other item in their offhand. As crossbows are not valid blocking weapons, you would not be able to block with this or wield a weapon in the same hand wearing this, and would have to take any hit that landed on it as if it hit your arm.
ARB said:A crossbow cannot be used for blocking. It is considered a Two Handed weapon for attacking purposes; in other words, both hands and both arms must be free to utilize a crossbow.