As Mike stated, there's no problem in putting #20 over the #26, because of the cleaners. It removes whatever is under it. I did notice that the #20 didn't last as long on top #7 as on bare paint, so maybe the cleaners aren't that effective on really oily bases.
As for how many coats, I like to put at least 2 coats on, partly because I believe there is some layering going on, but mainly because I feel more secure that I get better coverage, in case I missed any spots.
And as 4DSC stated, I have noticed that the shine gets better with multiple coats if there is some time between coats. And this is not an illusion, as other people have commented that my #20 results are better than theirs.