Awlgrip (and all two part urethanes that I know of) use agressive solvents that will tend to damage single part undercoats. Hence, these urethanes are usually applied over two part epoxy primers.
Like you, I have applied two part urethanes over single part undercoats sucessfully, but these undercoats have been older well-cured finishes. I don't think there's any risk that the finishes will deteriorate with time, as the solvents in the urethane will flash off fairly quickly and no longer be a risk to the undercoat.
My worst experiences with two-part urethanes have involved a) applying the urethane in the sun, causing the skin to set up before the solvents flashed off (giving a pebble finish); and b) application in humid conditions, resulting in a flat finish.