It needs its own UV space within the 0-1 area-- not stacked. So some shifting around of other UV elements/islands would have to be done to fit it in there. Even if the UVs are identical, if the texture needs to be unique it can't be stacked.
So, you're saying crotch-leg 2 has a separate material entirely than crotch-leg 1? I'm not familiar with UDIMs so can't speak to that part of your story, but for a traditional workflow, the 0-1 space (the lighter gray square where most of your UV elements are) is the only space that gets unique texture information-- it…