Acrylics can be sprayed on enamels no problem. In fact, they're almost preferable in that case since they won't react with/degrade the enamel the way a lacquer might (I stress might...).
Just be sure to leave the enamel plenty of time to gas out before sealing it. I have zero experience with Humbrol or Tamiya enamels, but Model Master enamels are usually good to go in 1-2 days. White Ensign, the other enamel I've used, can take as long as a week. Basically when it stops smelling like paint, you're good.
On the Bench: 1/32 Trumpeter P-47 | 1/32 Hasegawa Bf 109G | 1/144 Eduard MiG-21MF x2
On Deck: 1/350 HMS Dreadnought
Blog/Completed Builds: doogsmodels.com