Ok... my perspective is based on my own experience... take it with a grain of salt.
I was put in charge of my 2 siblings when I was eleven. The crazy things we did, mom still doesn't know about. We did dangerous stuff with cars, trucks and farm equipment (country kids)... but had we lived in town? We would have done dangerous stuff with neighborhood kids.
I vote no on leaving 2 kids home alone together.
Perhaps you can find a daycare that will allow one or the other to help out? My daughter's daycare did this for girls when they got to be 9 or 10... they got a small wage - a couple dollars an hour, and were put in charge of helping with crafts and stuff. It helped the daycare provider, and gave the girls self-esteem... but it was all done under the supervision of responsible adults.
The YMCA in our town has some excellent summer programs for kids... maybe you could stop by and talk to them about options available for your kids.
Camp sounds great, to me. Both my kids went every year and it is still the highlight of their memories.
You may have to make some hard choices about the houses. Letting one go may hurt, but if it frees up money & time for you and the kids... you can buy another one later.
Edited to add: I think when I was stressed a lot, I tended to try to keep the "status quo". I don't know, but it sounds a little like that, to me. Trying to maintain "normalcy" by keeping everything you have the same?
The truth is, the status quo is already gone. It may be that you can get out from under one home with a quick (very quick) sale... otherwise, you might have to accept that your lifestyle is already changed and just stop paying that second mortgage (if keeping the 2nd house is important to your AH, he can pay it).
(((hugs)))