I still here... understood 90% of it... :) I did not think about the multiple mill I have to say... in my case I just have 1 :)
A Boxford 260 VMC. I just don't have the physical space for more. (I had to take the 260VMC out of its enclosure to make it fit, BTW, that is not an easy task, as it is glue/cemented to the enclosure on the base. If anybody interested in doing the same, let me know I'll give you the procedure)
Would love a CNC Bridgy!

Back on tracks:
For the multiple mill we could use multithreading process using .net. We have a piece of sofware at work design to talk to 8 electronic devices on the same time via 8 COM port. It works a bit like your idea of a seperate program runing a loop in the background, exept it is an other thread / process in the main program.
As long as your CPU suppots mutithreading (I guess all do these days) then it should work...

RNR