"You can, however, put many[1] different GPUs in the same system, and then the onus is on the CPU program to properly manage and send work to each one separately. This is not SLI."
Interesting that you mention this as I've been searching for an answer before I build a ML rig and embark on learning it. Is there any benefit for ML performance if I put a GTX 970 and a 1080[ti|x] in the same machine? Just trying to figure out how to re-use spare parts.