Whether you make both companies aware is a personal choice. I only worked with companies who had absolutely nothing to do with one-another, ensuring they were in different industries. I also always made sure that any contracts I signed were vetted and that I was in the clear.

For me personally, I always made it clear that my time belonged to me and that I had other things going on. I set realistic expectations based on my compensation and met those expectations. It was never a problem for me. That would sometimes mean that I was working 20 hour days if there was a crunch or emergency at both places, but that was also generally followed by ample time off.