So my floor heat just has two loops and I was thinking that instead of buying a manifold and capping off the unused connections that I could just use pex T fittings instead? My main concern is cavitation...
You could do it that way but it doesn't give you the ability to adjust the flow.
Manifolds also typically will have bleeders and such on them.
If you do decide not to use a manifold I would be sure to leave room to put one in later.