DP9 and Loopback routing?

[John Kieltyka]

Apologies ahead of time for any protocols I might be missing...

MacOS High Sierra 10.13.6
DP 9.52
MOTU 16A interface

PROBLEM:
I'm trying to route a multi-track DP project to Zencastr, via Loopback, in order to allow a guest to hear everything, while simultaneously routing that guest's audio back into DP, where I hope to record it on its own track.

SETUP
1. I've got a project open in DP, with multiple tracks.
2. I've got Zencastr open in order to record guest audio.
a. Inputs and outputs are set to the Aggregate Device
3. I've created an Aggregate Device in Audio MIDI Setup
4. I've set Loopback to the created Aggregate Device

PROBLEM
I can get my audio into Zencastr via Loopback, OR I can get the guest's audio into DP - for the life of me I can't get BOTH

I've searched this forum, as well as tried setting everything to JUST the 16A (not the Aggregate Device). No luck.

Any help would so much appreciated. Thanks!