Multiple transfers can run simultaneously, up to the limits of max connections (in rsyncd.conf) and your system. Really though, it makes little sense to fill memory and thrash your disks between just two boxes. Better to do them serially as you are now.
I spoke earlier about not publishing an entire drive, but I'll go against that for now. You can make your setup simpler by backing out the module one or more levels. e.g. Change the module path for [test] to /cygdrive/c/Users/John. Then, in your rsync command, use:
rsync -avP --delete --chmod=Fo-X 192.168.1.152::test/Downloads/ /mnt/user/photobackups/Downloads/
If you plan on more than a few paths, better yet would be to put it in a loop using include-from to pull from a file containing your list of backup sources. Then add exclude-from to filter out the unwanted.
Echoing myself, you might also look into the -b and --backup-dir options which make versioning simple. Create a timestamp, use it in a directory name for --backup-dir, and anything that's changed or been removed will be moved there instead of being lost with the --delete option.