Atty, here's hoping you can get rid of those bees without too much aggravation.
It would be real easy if either nest was on the ground since we could try the DE. It's not a bad idea at all. I think it only works by them walking through it and getting it all over them in large quantities.
So I'm not ignoring your advice, I just don't think it will work for these locations (1 inside wall, 1 in a heater base where I can't get at it).