Generally speaking, you don't want to apply LACQUERS over ENAMELS. This is because the solvents used in lacquers are MUCH "hotter" (stronger, faster evaporating) than enamel solvents, and the lacquer solvents can 'dissolve' the enamel paint underneath them, leading to "alligatoring" or cracking or other things that TOTALLY ruin the paint finish. When something like this happens, your best bet is to wipe the whole mess off with a paper towel soaked in lacquer thinner, allow it to dry thoroughly, sand anything else off if necessary, and start over with the finishing process...
Now, like everything else, there ARE exceptions to the rules... some folks HAVE applied lacquers over enamels successfully, but it is NOT a "beginners technique" and is risky at the best of times... it can be done in VERY VERY THIN, gradual coats, that don't put enough lacquer thinner/paint onto the enamel surface to attack it.
Enamel, having 'softer, slower' solvents, CAN be applied safely over lacquer in most cases. Other paints can be applied over one another with various levels of success...
Paint formulations have gotten SO complex and different between manufacturers that it's hard to say anymore WHAT is compatible with anything else... for instance, many folks report problems with new Krylon even being applied over itself, even after CAREFULLY respecting the recoat instructions on the can... other paints seem to work well no matter what they're applied over (and usually with just about anything applied over them.)
The main thing to remember is, no matter WHAT you're using... the safest bet to ensure compatibility of the paints is to do a PAINT TEST on scrap tubing or cardboard or something similar to ENSURE that the paint coats will work nicely with each other and not attack each other...
Good luck! OL JR