You'll get better answers to JP1 questions if you ask in the JP1 forum:
[Link: hifi-remote.com]

You could use the "Device Combiner" protocol to create one upgrade merging two different JVC device numbers.

It is probably easier to use two setup codes. Whichever device number has more functions should be used for the setup code actually assigned to whatever device key you use. All the commands for the other device number could be defined as KeyMoves from a second setup code.

When you look for an existing upgrade in the JP1 archives, you don't need an exact match on model number. The upgrade JVC RX-8020VBK.txt is a good example of using the Device Combiner protocol for JVC, including the two device numbers you wanted. You could probably create a Device Combiner upgrade for your model by making only minor changes to that one.