How to tell my current project adviser that I want to try working with someone else next semester? I have been working with Professor 1 for a semester. He has been very helpful and we have spent some time together this semester discussing problems. I enjoyed working with him and also find his research interesting.

However, there is also this other Professor 2, whose work I find interesting and he works in related areas. So, I want to work with Professor 2 for a semester before deciding whom to stick with. 

Meanwhile, I want to continue the work that I have initiated with Professor 1. I want to convey this message to Professor 1 in a manner that will leave the possibility of working with him after next semester open. So, how should I tell Professor 1 that I want to do a small project with Professor 2 to try out things, without sounding rude?
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.