This is a hard question to answer; there are many possibilities and it is hard to nail down which one it is. The trees still may not think they are mature enough to fruit. This is a problem I have on many varieties but I don't have your two so I don't know to what degree it is a problem for them. You may in fact have two of the same variety to due to a tagging error (far more common than you would imagine), so there is no cross-pollination. Lastly, the trees could have experienced colder weather than you thought had come through due to e.g. being in a frost pocket. I don't think the bees or the crabapple is a problem, because you should have at least gotten a few fruits if there were too few pollinators -- the odds of getting nothing at all is very small. If you want to eliminate that possibility next year, hand pollinate a few to see if it makes a difference.

Scott

I had not even considered that these might both be the same variety!

I'll try the HP thing next year.

Many thanks!