I also tried in the past a few times, and had several failures. This is the first time it seems to be going well!
I don't think feeding them would improve things, but you might as well try as a last resort.
I think them being two might be the missing thing. I am curious to see if they're going to stay a peaceful couple when the first workers hatch, or if they will follow the Lasius way and "naturally select" a single queen.
futurebird