Not a bad idea. I was thinking along the lines of using the 20mm for bombers or other tougher targets, MG alone for fighters. Same reason the Spits did (isn't it?).
The mixed armament in most Spits wasn't to use against different targets. One of the reasons was a difficulty in supplying enough hot air to stop the guns freezing at altitude. The Hispanos liked to be kept toasty warm the Brownings needed a lot less warm air.
Spits from the mkV onwards could carry a mix of guns and up to 6 cannons but that was only ever flown to see if it worked. 4 cannons were most often used in the Med region but even then weren't that common.