How to tell my current project adviser that I want to try working with someone else next semester?
There is a famous quote "The bitterest truth was always better than the sweetest lie". You should always the truth, just tell the whole stories about how you are interested in prof 2 but still want prof 1 for your current project that you initiated together. As a professor, he is a professional academician, so he should be able to accept whatever you said as long as you convey it truthfully. Academician always know that working with different people is always better than sticking to only one. Do your best and good luck.