I dont 'think there should be any problem if they are using different OS....
But....
I upgraded my 5,1 12 core to Mojave this weekend and wow... It scored 20% better on Geekbench right off the bat and VEP7 runs really really well on Mojave, not so great on High Sierra... Part of the speed increase is probably precisely because of the new Metal card offloading some tasks to the GPU, but geekbench measuring 20% better score has simply to do with multicore performance...which somehow must be more efficient in Mojave compared to Sierra that I was on before.
I can really reccomend Mojave. THe RX580 or even RX560 cards are not too expensive, I found my RX580 on sale for under $200. As it turns out that video card also enabled nicer video modes with my 4k monitor too, but you probably don't care about that for your slave, that's probably part of why you don't want to buy a Metal card for it too, I get it, but that is the future for the 5,1 MacPro. When you go through the process of upgrading you will also upgrade your firmware which will make it possible to boot from NVMe drives too should you ever want to.