SPOILERS FOR AOT S4 AHEAD
Eren was able to have multiple titans and merge them, he had the Attack as his base titan but was able to use all the powers from the warhammer. So I propose if one human has multiple titans, lets just say warhammer and attack, one titan could use both powers. The main appereance would be from the first titan they aquiered but it could have some kind of selection menu to choose which power out of all the avaliable ones to use on each hand.