I do them by making separate prints and mounting them together - a whole nother subject.
The last time I did what you are trying to, I made the test prints for the first one (whichever is most critical), being sure to make a final print of the edge that will adjoin the other one. That is, if you start with the left side, be sure you finish with a long vertical "final" strip of the right side of that image. Then make the final exposure on the final paper, covering the right side, as suggested above. Then put that final away. You will need to mark somehow the corners of that image (little dot impressions with a pencil).
Then do the testing for the right one, including a final strip of the left edge, and make sure it conforms with the right edge of the left image. Then put the final print in, covering the left side, and expose.
I found that doing all the testing with both images, then the final image had too many issues, what with changing negs, etc. Also, I found that exposures were more reliable right after testing, than done later, but I don't know why.
I did a print once with 9 exposures, making the final print a good hour or more after the first test, and the exposures were all over the place.