I build more than 1 model at a time.
That way there is always something to do in my 1-2 hour sessions when I actually do have a chance to build. I have found that I can paint a lot of things that need to be black in one session, and then go do completely different parts , trimming and painting those.
After all, that is how the real things were built,,,,engines here, tires mounted there, bodies and frames there, bombs shipped in from,,,,,,,,,,,,you get it.
I do concentrate on just one kit when it gets to the point that all the painting is done, and it is time for final assembly after decaling. That way I don't end up with Skyhawk landing gear on a Crusader or something, lol.
Rex