How can Titan Shifters with multiple forms control which form they transform into?
Actually having that titan just means that the shifter has thar titan's powers. So if a shifter has multiple powers in possession, he/she will still just tranform into one body, and can use the titan power.
A little spoiler but eren will still have the titan body like he has now, and it has been regularly mentioned that he can use the war hammers power, so it doesn't mean to have a different titan body per titan power.
A Spoiler from Chapter 117 makes it clear, even if you are anime-only, it will only grow your interest, and not spoil the storyline likely -

 

 In this, eren is using the war hammer titan's power, in his usual form (the one he had earlier too)