Sounds like it might be more of a planning problem than a code problem.
If the two betas are competing - they are not really betas, they're proposals. If people can build directly from branches to test them, do that (remember you can spec a full-path git URL with #branchname instead of a version number). If they must be released to test, maybe do them as two separate alpha releases published from branches. Regardless - you will need to communicate how the decision gets made and when/how one gets merged to master and released in an official version.
If they're just two sequential features or updates, do two releases. Work out a roadmap so everyone knows what's going on, then follow it. If they can't be released separately, just don't release until they're both done.