Ethics of "Double Dipping": Helping 2 professors with research during the summer
This shouldn't be a problem. The professors aren't your employers who demand 100% of your time - they might not even be paying you - so they are not likely to expect you to commit to their lab and their lab only (it'd also be unreasonable if they expect this). Meanwhile by working in two labs you're maximizing your exposure, which can only be a good thing for you.

The only serious consequence I can see from this arrangement would be that you might not be able to commit enough time to actually achieve something substantial. If you aren't going to get any real research done in both labs, you might as well commit to one lab only.