If you want to constrain some versions between branches, I think it's up to your plugin to define the right dependencies. If you use `mvn install`, then you break isolation and you let p2/tycho decide what it's going to use. "Porosity" is a risk of `mvn install`; using it has always been discouraged by Tycho developers since Tycho 0.1 and has been discourage by Maven developers has well for several months now
If you need to orchestrate builds, then either you use "mvn install" but with different p2 repos to have some isolation; or you can just not use "mvn install" and build intermediary p2 repos (1 for each Git repo) so the build can interoperate by reference the p2 repo from the other repository, or you can have a super pom to build everything at once.
I really think that both p2 and Maven already have some means to provide some isolation that you're looking for; so I don't think Tycho needs to change here.