WOW - almost $4,000 per month on mortgagaes..... yikes.
Personally, I would put the $500 to the higher interest mortgage. Get that mortgage paid off before the 15years, then turn around and pay the extra $500 to mortgage #1. Do this only if you already have a good nest egg for a good couple of months because you gotta be able to cover expenses should one of you get disabled and cannot work, or whatnot. Once you got a good nest egg, then pay extra to the mortgages!
Hope that helps!