I suppose I could rent a double and then if they don't fit they can take turns riding.
I was going to bring DD1s stroller anyway for the airport and to have around the hotel and for shopping etc, so it wouldn't be so much trouble to bring that one on the buses and just have two in the parks.
So, I think I've just answered my own question - bring DD1s stroller and rent a double

Typing the question in just makes you think better!