Split CNC controller? Fadal + Mach3

I just purchased a 3 axis Fadal 15RT with the 88HS controller. I had been looking into more of a DIY solution and so had picked up some things to run a mach3 system. Now I am wondering if it is possible to run 2 controller at the same time? I am wondering if I can throw in a 4th axis stepper motor like this: 4th Axis Hollow Shaft CNC Router Rotational A Axis Φ1MM 4 Jaw Chuck Engraving | eBay

I am using Bobcad and figured maybe if we are drip feeding both controllers and maybe it is setup where I send the same instructions to both controllers at the same time that maybe the Fadal would ignore all instructions to the 4th axis, and the Mach 3 would ignore everything to the other axis since nothing would be set up on them.

I am pretty new to this, and maybe it is a stupid question, but it would be cool to be able to add a small 4th axis for less than $3000.

It would be extremely tricky to get this work right. Just syncing at the beginning of the program would not be enough because you would need to account for the acceleration and deceleration motion profiles of both controls.

I just don't see a simultaneous 4th working like this. A 3+1 has been done and HAAS was a big player in add-on 4th boxes for this purpose. The synchronization was done via M-codes to perform indexing.

If you are really wanting DIY, why not pick up a 4th axis card, drive, and motor for the Fadal and homebrew yourself a 4th?

I know this is a super old thread, but I also have a Fadal VMC15 that I was looking for a similar solution. I found this project on Github that looks promising to me: GitHub - svenhb/GRBL-Plotter: A GCode sender (not only for plotters) for up to two GRBL controller. SVG, DXF, HPGL import. 6 axis DRO.

It seems it can control 2 controllers like we want. I am going to try using steppers for the 4th and 5th, and let the other controller keep using the servo motors they came with.