When doubling up stereos like that, heat is definitely going to be worse than it would be if it was just the 'peg.

If it were me, I'd make sure you've got a cooling fan in the 'peg.

The over/under order is good, with the 'peg on the bottom. If there's enough of a gap above the 'peg so that the cooling fan can exhaust, you're good. Check for that.