I'm building a router with 2 Z axis.
I plan on using SwapAxis() during toolchanges, based on the tool number. Numbers under 100 will use one spindle, and numbers over 100 will use the other. I think you also need to use G52 for the second spindle offset. SHould be able to handle this in the M6 macros.
It can be done, but there possibly be a lot of macro programming involved, and as was mentioned, you'll be basically on your own.