We have had multiple production and testing systems in the same DS8950. GDPS HS does not effect the systems. As far as I know, softfence is done on a device level. If you plan to use a part of the devices in an LSS in 1 system and another part in another system, I would expect it to be fine too, but that we did not have. (We only used/shared parts of LSS ranges across unmirrored k-systems.)
We also had DS8910 with multiple sandbox systems. Never seen any impact across systems.