Les deux passent la nuit ensemble.
Les deux se passent la nuit ensemble.
I was always confused on the difference between using se or ensemble.
I think an easier way to understand the difference between se and ensemble would be with a simpler sentence :
They would translate respectively to :
Does that help?
Quite often « se » means « self ». So you can think that it stands for object of action:
Well, the translation is rough, but it's aimed to be illustrative ;-)
Another use of « se » is to express idea of return action, consider this example:
Also « se » may be used to create passive voice constructions: