Big difference.... and it's speed...

Today's hard rock, metal, require fast drums 'rolls'... I call them rolls because they are that fast these days. 2 is easier... although there does exist a few who can play a single bass drum so fast you'd swear they were using 2. But also:

They sell double bass drum pedals... they strike only one bass drum but there are two pedals... so same thing.... only two two bass drums look more bad ass.

I have a double bass drum pedal but never took the time to get good with it.