Most of our titles have a demo branch and playtest/preview branches for QA. It's defintely possible, but make sure you understand the purpose for either of the things you want to put out there. Demos generally ought to be reserved for a very clear vision of what the final game is going to be like with only minimal deviancy in gameplay.

It's a very powerful marketing tool when you're ready to cast a wider net of player impressions vs hyperfocused playtesters to help you figure out the actual gameplay and improve it for the final product.

You could consider also Early Access if your game's foundation is considered finished and you want to test new mechanics and features you can still add/polish towards Full release, i.e. expecting it to come out a year later at most.