If their output is at the same GND (as you descibe), just use one ground plane for everything.
Regulation will be referenced to that GND, which should be the same for both. If you split the planes (and don't have a connection on your board), the wire resistance from the 48V will induce some (probably small) difference to these two GNDs, if your converters feed two isolated circuits, this wouldn't matter, but if your converters feed two voltages into a circuit with common GND, you don't want this difference. Additionally, you'd create a GND loop then, this isn't a good idea EMC wise.