Well, son of a --
Get a load of these everybody. Four tests, with the following specs:
Test 1: Dario doppelganged, third party slot
Test 2: Dario doppelganged, second party slot
Test 3: Two texture enemy reference
Test 4: Two texture enemy reference with Dario doppelganged
Looks like there's room for up to five full 128x256 textures in VRAM. The first three slots are allocated to the player's party, the last two to the enemy party. When Dario is placed in the third party slot, his texture erases the enemy's texture. When he's placed elsewhere in the party, his cape annihilates a team member's texture.
I think there's hope here -- as I illustrate in the fourth test pic, there may be a sixth texture slot that can be written to in battle, with some tweaking to whatever code tells the game engine where to read and write in VRAM. In Test 3 I didn't show it, but the Dream Devourer's texture overwrote some overworld stuff in VRAM. If the sixth slot is also overworld or otherwise non-battle stuff, it should be possible to move the read/write position for enemy textures over one slot, making four for the party and two for the enemy. The upshot is, it would probably be impossible to have more than one dual texture character in the player's party at any time once the game code were altered. For playable Dario, this type of alteration to the game code will be necessary unless it's just Dario on a solo mission (or possibly with one other character, haven't tested that yet).
Here's the actual VRAM dumps if anyone wants to examine them firsthand.