1st post ?
And dual spindles .. well - good luck.
There are any nr of solutions to do this, and all are somewhat complex, and all are somewhat costly.
Mach3 will not support this natively, and Mach4 can support this in lots of ways, depending on the controllers.
Whats your budget ?
Ie you are talking about something complex - can you afford good controllers ?
How much do you want to do in hw ?
What are the goals re: reliability, accuracy and crash detection ?
Errors with chips ?
Do you protect the other spindle, or swap it out ?
With what hw ?
Any nr of servo drives can slave axis themselves.
So can some controllers, ie motion-control engines.
Again, these are easily 1000$ per axis (servos) or about 1000$ for a good 6-axis controller.
Mach4 will/may be able to drive multiple controllers simultanously.
Again, your expectations and budget will have a huge impact on the best path.