My wife insisted on one small valve. I had a few from old bathrooms like the pair I put in my head(they work great, very low flow due to narrow stem and many turns for full open). I thought, ok, I'll mix the hot and cold after the shut offs, then connect to a single.

Got a few connections to combine the supply lines, and then went to attach. Turns out the threads on some of these old fixtures, were built by the manufacturer and of no common thread size. I found that out from a good plumber friend. He also told me, all kinds of evil stuff would go on in the house with the hot and cold mixed that way.

Looking at single handle mixing valves, none of them were even close to being small enough to not make the sink look like a finger bowl(it's only 8 or 9 " dia). If we find one, the water supplys are both below.

But as things get complicated in my house, my complicated wife said she was fine with hot only. She said she hates it when the kids brush their teeth in any sink they walk by.

Brush your teeth in hot water, no way,.... Brilliant.