thanks for the info - my process seems a lot longer and more complicated.
1. open image a and b
2. copy and paste a onto image b
3. use eraser tool on parts i want to bring through
4. flatten image and save as a tiff (call it image d).
5. open image d again and image c
6. copy and paste image d onto image c
7. use eraser too (same as no.3)
8. repeat process 4 and save as jpeg if you're uploading to web

hope it makes sense. nice to know there's more ways than one.