What's the difference between spolu and dohromady?
more or less none. At times it is more common to use one over the other, often they are interchangeable.
Well, "chodíme spolu" (we are dating) can NOT be changed into "chodíme dohromady". Otherwise they are pretty close.
Though I've never found a difference, 'dohromady' is the word always used in a pub or restaurant for paying together.