That's a sticky one. Seen it several times myself. I bet a fly on the wall in their house would hear some interesting conversations between those two!
You two need to have a very open discussion with them. Explain your concerns. You may een want to take H out for a guys night and talk over beer withoutt he aldies present; while that goes against the policy of 'open communicatio'that is so necessary to make these this really work, you might get some things out of him that he wouldn't be comfortable discussion with B around.
Be that as it may, all these concerns need addressed before things get beyond a casual encounter or two.