Yes, it sounds coherent, and it is better than what I suggested.
If A1 is the original reference, E1 any electrode and A2 your other reference (recorded with A1 as the reference):
E1 - 0.5A2
= (E1 - A1) - 0.5(A2 - A1)
= E1 - A1 + 0.5A1 - 0.5A2
= E1 - (A2 + A1)/2
= E1 referenced to the averaged of A1 and A2
Please not that this is a forum to ask questions about the Brainstorm software. I would be happy to help you using this environment.
I’m not an EEG expert, so for pure EEG related questions I suggest you ask questions to a more EEG-oriented community.
Cheers,
Francois