can dogs have a litter with more than one dad.

I bet it had two parents. Call it a hunch.

I suppose there could have been some sort of dog orgy with simultaneous orgasms.

Yes. Usually the bitch releases several eggs which can be fertilized independently.

There are cases on record of human "twins" with different fathers.

mastiff and rednose. holy dear god its going to be a monster... at 3 months he is almost half the size of his mom.