OP
In an interesting twist, I have a Tec40 student next week who has been using the halcyon 40 for diving with a single tank. They will be using the same bcd for backmount doubles.
Apparently he has no concerns using that wing for a single tank. I became curious and looked it up on the halcyon website. They had this to say:
“Two different lift capacities allow easy pairing with small doubles or large, heavy steel tanks.”
The wording is a bit ambiguous.
They told me it's safe but would taco and be difficult to control. I wonder if the single tank eclipse is the one they mean.