Two computers, one Petrel, one Perdix.
Perdix set fairly conservatively, Petrel much less so. The Petrel is the "get me out of the water without getting hurt too bad" computer, and the Petrel is the one that controls the dive, unless it ever fails. If it does ever fail, the Petrel allows me to change the GF high to be more conservative underwater so I can continue the ascent as usual.
I assume that you mean that the Perdix controls the dive.