Has anyone here built a 2 spindle (or more) CNC machine?

I saw a custom machine for sale on eBay which had both a high speed CNC router spindle and a seperate belt-driven low speed / high torque milling head:

https://www.ebay.com/itm/CNC-All-Bal...8AAOSwX61ZCfsI

The listing states that the machine is capable of cutting both hard metals like steel and titanium or high speed routing of wood, plastic, soft alloys etc. It seemed like a good idea and I was wondering why we don't see more like it here on the diy end of the market?

I have seen others that seem to have 3 or 4 identical spindles on their gantry. Can machines be set up to cut 3 or 4 (or more) identical mold cavities (or other parts) at the same time? Or can multiple spindles be used as a cheaper ATC alternative? You could buy 5 or 6 Chinese spindles for less than buying a single Chinese ATC spindle without a carosel or air source.....

I like the idea of having one machine with multiple capabilities. Many people have space and budget limitations. CNC routers, mills, 3d printers, CNC plasma cutters etc are all based on an XYZ computer controlled moving frame. It seems like a no-brainer to build one machine with two spindles, or one spindle and a 3d print head etc over the alternatives.

Is there some limitation in software like Mach3 or some other limitation that prevents people building multi-use diy machines or machines capable of making multiple parts simultaneously? Or is it just a case of making sure your steppers are powerful enough to move two spindles and making a wider Z plate to accommodate them?