Could conducting collaborative research with more than one professor BEFORE applying for a PhD lower an applicant's chances of acceptance?
I don't see any problem with adding additional collaborative projects, but make sure you can finish something and aren't stretching yourself so thin that you make no progress.
There's nothing wrong with exploring research interests, if anything, it's a benefit. But you also want to show you have some follow-through. It's completely different to have started 10 different things and finished none of them than if you've completed some projects and also tested the waters on others.