Mike,
Here is (hopefully) a tangent out-of-the-box thought...
How about using the wing to blend the 2 gases before breathing it into your lung ?
Some people already learned how to breath off the inflator in case of emergency (IIRC, you have as well), just don't breath it back into the lung.
Use the wing as a blending accumulator and you could learn beforehand what squirt ratio between the 2 tanks gives what fiO2. Or it could even be estimated on-the-fly knowing the fiO2 of the tanks.
The tricky parts are:
- The tanks might not have the inflator hoses connected.
- Buoyancy issue with fluctuating wing volume, but might doable if having access to the downline or cave roof.
Just some food for thoughts...