I am trying to make multiple .deb's from a single source, but would like to have different maintainer scripts (pre/post inst/rm) scripts as well as different systemd unit files for each package. Is this supported?
To have different maintainer scripts for each package, you add a prefix to the script. For example if your packages are called mypkg & mypkg-extra you could create these files in your /debain directory
debian/install #Files to be installed in default package.
#This must now exist even for your
#default package, or your files will be missing.
debian/mypkg-extra.install #Same, but for second package
debian/preinst #Preinstall script for default package
debian/mypkg-extra.preinst #Preinstall script for second package
debian/mypkg.service #systemd unit file for default package
debian/mypkg-extra.service #Systemd unit file for second package
Also, I believe the default scripts/unit files could be prefixed by the default package name as well, but I didn't try that.