Peter is unfortunately right. See it this way Richard, you're most likely correct but like Peter said AncestryDNA doesn't provide us with the tools to verify it properly. Gedmatch on the other does it.
If you have paper trail connecting both then I would go all out until I have the triangulation. Even though at 5th cousin level it can happen that the ancestral DNA segment has vanished at all. But in these cases it helps to lower the criteria and prove that they still triangulate, though below the standard criteria.
You will do someone a big favour if he/she matches into these two people. They have a proven (if triangulation works out) MRCA to start with, much easier to then attach their family tree to it.
I only have one case as such and I'm still trying to figure who of the ancestors is the right one but we're getting closer as we have lots of other matches that don't triangulate with me and are obviously a match to those two on another ancestor of the MRCA.
It's like a giant puzzle piece, so much fun when one difficult piece fits finally in. Keep on going!