DS and DD are two years apart (7 and 5), and either fight or play. They never leave one another alone! The playing is a recent development; there was a phase when they only ever seemed to fight. I put a vast amount of time and energy into getting them to play together, and I'm hoping that it is finally paying off!
Tips:
Take any possible opportunity to get them to do or say something kind to or for each other. If need be, create the opportunity yourself.
Take - or, again, create - any opportunity to get your DS1 to see DS2 as a source of fun and entertainment. When DD was about ten months and crawling, I invented a game called 'slip-slap': DS and I had to hide under the table from DD, who was 'slip-slapping' along the wooden floor. DS (then 2.10) thought this was hysterical... He also thought it was hilarious when we used to put her on the bed with a heap of cushions and count how many seconds she could sit up for (so she must have been very small at the time!) Silly stuff like that seemed to help them to see one another as playmates rather than rivals for my attention.
Encourage them to play board games together (of course, two is still young for this - but you can 'help' both of them).
Do things like make huge pictures where each child has their own 'section' to draw/stick/do whatever on, then hang up their joint masterpiece.
Engineer situations where they can conspire against you!
Mine still fight something rotten, and I spend a lot of time doing the very dull thing of trying to help them to sort out their problems themselves. But as I say, they do often play very nicely now, especially if it involves dismantling the house. Good luck!