I asked Sequential Support about this, and I'll put the information I received here, in case anyone else needs it. All NRPN numbers and ranges are in base 10.
Layer A/B Select
NRPN: 80
Range: 0-1
Type: Enum (Layer A, Layer B)
Notes: She did not specify what value corresponds with what layer, but 0=A, 1=B is a strong likelihood
Bi-Timbral Mode
NRPN: 89
Range: 1-3
Type: Enum (1=Normal, 2=Stack, 3=Split)
Notes: The structure of this enum contradicts Sequential's normal 0-indexed praxis. I assume that 0 would also be treated as "Normal" because that's what NRPN 89 is for all Prophet-5 programs.
Split Point
NRPN: 95
Range: 36-96
Type: MIDI Note Number
Notes: This note number range covers the Prophet's keyboard range.
Layer B Program
NRPN: 91
Range: 0-39
Type: Same as sysex voice message program number (0-7=bank 1, etc.)
Notes: Apparently there's no NRPN for group number. She says:
"Changing the Layer B program involves two steps:
-Change bank/program: NRPN 91, value = 0-39 (0-7: bank 1 programs 1-8, 8-15: bank 2 programs 1-8, 16-23: bank 3 programs 1-8, 24-31: bank 4 programs 1-8, 32-39: bank 5 programs 1-8).
-Change group: set CC 32, value = 0-4 (Group 1-5)."
I don't really understand, then, how group number is stored as part of the voice data. I'm assuming that the Layer B program must live in the same group as the Layer A program, though, as CC32 is just the normal bank change MSB control message.
Layer A Volume
NRPN: 88
Range: 0-127
Type: Integer
Layer B Volume
NRPN: 96
Range: 0-127
Type: Integer