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…
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.