Solution:
There are several ways - in some cases the functions inside a Power Manager may be stand alone and not affect the other device controlling different supplies. In cases where there are zones or sections that need to get powered up and down, tie an output from device #1 to an input of device #2 and another output from device #2 back to an input of #1.
When #1 is ready and it has sequenced up its supplies, it can send a power-good signal to #2 to start its sequence. If there is an error or fault from a supply on either 1 or 2, the signals can talk to each device and shut down or send an appropriate error flag.