It's been decades since I did any but, I do remember "mapping" the finished image being a big help.
1) Focus the main negative onto a white sheet of paper. Trace the outline of the image part you want to use. Mark the enlarger column height with a piece of tape, or marker.
2) Repeat the process with the other negative(s) for the finished piece, remembering to mark the enlarger column height.
3) When you get ready to make the print, make sure the paper is all the way to the right (or left) in the easel. Also, have an empty paper box to place the paper in between exposures, so you can focus the next negative. Just make sure you place the paper in so it is aligned the correct way for each subsequent exposure.