I'm assuming you're talking about explosives.
I'm not sure how it would actually work, but I would probably increase the radius by 2" for every 3 or 4 explosives and then continue as usual.
So if there were 5 MK6's exploding in the same place, it would have a radius of 12" for them being MK6's, then another 2" because there are another 3, for 14". Then you roll all the damage at once, so 30d10s, and take away the lowest for each 2".
As for the second one, I would say one explosive would have to roll damage against the structure level of the others. If it breaks it, the second explodes as well. Anywhere the explosions are overlapping, the damage overlaps as well.