I think if it is not solely theirs, then the whole post doesn't apply. I also think that there doesn't need to be a boundary if the other person just realizes I cannot wear a small shirt because I am a large body. If they start to put the t-shirt on then that would be a serious cause for concern.
Quite interestingly I did find an AITA post about a guy who's late father passed a way and passed down his luxury car. To the guy it was the embodiment of his dad. He didn't want anyone to wreck it as it would be like his dad passing away a 2nd time. He got rated the AH
AITA for not letting my wife drive my car ? was the thread title
So I guess if the other party passes away, then by AITA standards, ownership has passed down even beyond metaphysical meaning.
I guess in pure theoretical standards if half of the shirt is owned by the partner, it wouldn't make much sense to put half of a shirt on. It just doesn't work in any way.