Force Roto Brush to see objects, then multiple/subtract masks for grading?

First time using AE for masking so sorry if this is basic.

We use mattes to adjust colour, not to cut out objects. We'd prefer to track just a few key objects, then combine or subtract those mattes to create alphas for grading the rest of the elements.

e.g. Consider this shot of bed sheets and an arm...

We can create masks for the sheets and the arm, but Roto Brush either ignores the arm but misses the overlap where sheets go behind it—or tracks the arm which creates missmatched alphas between the sheet and arm alphas, causing halo artifacts after grading.

Questions

1. Can we teach Roto Brush to see the sheets as an object, then subtract a separate arm alpha?

2. Or is there another tool to mask-out the arm from the sheets Roto Brush - and if so, whats the best tool to use?

Also, we've only just started playing with Roto Brush, so layer-wise, do we create the masks on multiple instances of the same clip (on differtent layers), then multiply/subract layers to create the masks we need?

Hope that all mkaes sense, any pointers in the right direction much appreciated.

Cheers, Ben