Okay I got you, it will be a few more steps then. It's basically what I described above, but you need to use an intermediate mesh and select the vertices on the source mesh when you do the copy weights. Remember, copy weights can work on a mesh selection or a vertex selection between source and target.
So duplicate your T-shirt and undershirt meshes, this will unbind the mesh. Delete all the T-shirt faces leaving you only with the undershirt mesh. Bind this mesh to the same joints as the T-shirt/undershirt mesh. Select all the vertices for the T-shirt mesh which is correctly weighed and then select the vertices for the undershirt only mesh and copy weights with influence association set to Closest Joint. It can be helpful to select the verts by uv shell sometimes to make it easier, or make a selection set etc so you can get this selection again easier.
Clean up the weights for the undershirt and you could always leave it like this. Or do a final weight copy by selecting the verts on the undershirt only mesh, then select the verts on the combined mesh and do a weight copy with influence association set to One to One for a precise copy.
Where I work we have a script for separating and combining meshes while retaining weights. Very helpful for stuff like this!
