so you need to use tools that build both for you without you having to do either one manually.
I understand that the Suse buildforge does this for you, and I believe that the code to do this is available.
plus there is the alien program that will take a .deb and convert it to a .rpm (and vice-versa)
so the cost of creating both formats (if you don't care about the distro specific policies) looks like one line in a build script. hardly a crushing load.