All controlls seems to work with one "target IP" for the controlled switcher. If running more than one mixer in one network, they must have different IPs.
I think, it schould be possible to solve the Problem by software. sending the same command to both mixers and recceive the answers form both for synchronizing. Maybe a microcontroller can do this job.
Ask Kasper Skaarhoj (http://skaarhoj.com/). He is a ATEM and microcontroller specialized develloper.