2spindles/6stepper motors

[freddi]

Mach3 or grbl in terms of software? And why?
For a 6 axis cnc machine that is dual spindle and must be capable of double sided fsw and isf.

[Ger21]

Mach3, because GRBL has a fraction of the features that Mach3 has.
But I won't use Mach3 on any new machines, as it's obsolete imo.
I'm using UCCNC going forward, including on a dual spindle router I'm building.

I'm not aware of any low cost control that supports dual spindles, so a bit of customization is required, depending on how you want them to work.