Free: Contests & Raffles.
I think you would be fine just insulating the dog house. 2 dogs with shelter snuggled up to one another will keep warm enough. Some kind of clear dog door/flap would work great to really dampen the air flow.
Mine stays in the house year round
My lab lives out side 24/7 and is bedded in straw, in a igloo dog house like your set up.
Yea both of mine stay in their house I just sleep there and pay the mortgage.