Let me preface this with info that I have twins and have never owned a single stroller, only doubles.
Last year my considerations for 2 singles vs 1 double were:
2 singles are easier to maneuver
2 singles seem more compact on buses
2 singles allow the kids to get physically away from each other when they are annoying each other
1 double isn't so big and bulky it's hard to get around.
1 double when it's folded is a bit burdensome, but maybe so is juggling 2 singles with only 2 hands?
Here is why we picked a double:
1 double allows one parent to easily travel with 2 kids. Not only in cases of one person going and grabbing some food, but, in cases of one parent going by themselves somewhere. We didn't plan to need this feature, but, sure enough, after spending 8 days in the cesspool that is WDW my husband had to go to the ER and tested positive for Influenza. On that day, because I had the double, I was able to go to a park and enjoy it.