Is it recommended to separate personal and professional projects into 2 lists or can both lists be combined?
It's generally recommended that we keep everything in the same system, but there's nothing wrong with having separate project lists within that system. In OmniFocus, I have a folder of Home projects and a folder of Work projects.
There's also nothing wrong in having 2 systems - work and home - if your work does not allow you to mix personal and professional info in one system (for example in military and government institutions).
The price you pay is some inefficiency and two Weekly Reviews.