I also think its a little unrealistic to expect a 9yo and a 5yo to play together. I would have thought the 9yo would get abit frustrated by the 5yo and the 5yo might get upset about notbeing able to keep up. Also it sounded like when they did play, you still found it annoying. I would have thought playing football and playing spying seem fairly innocuous but if you want them to play something different you might have more success getting them absorbed in an activity then letting them carry on with it. Or set them a challenge to keep them occupied with something - how big a Lego tower/rocket/etc they can build. But it may be that you have to give them two separate activities to play with. It's probably hard for them to come up with things to do together that will keep them both amused that aren't boisterous. It makes sense that they would play more active games together as developmentally they will struggle to play thinking/fine motor/language games together due to the age gap.
I have DS(9) and DD(5) - they don't play together at all...but neither do they argue. They re in fact very close and will talk for ages, DS taught DD to read as well, but they have no 'play' interests in common. I have no problem with that at all, they each play their own games (either alone or with me/DH) and that's fine.
The older pair of my four hated each other pretty much from the day DC2 was born. Relentless, awful arguing. The younger two are really close and very, very rarely squabble - so really I think a lot of it is just down to luck.
But with my arguing pair - I often just walked away in an attempt to get them to learn that they started the row and so they'd have to resolve it. Once they realised this, they did seem to start fewer rows.
DC are 9 and 5. They can get on sometimes but when they argue they argue A LOT. Staying in with them a nightmare. Today we went into town in the afteroon then came back. DH and I read the paper for a bit and asked the kids to play nicely. They then screamed round the house, played football, spied on us, and generally were really annoying. IMO they are too old for this behaviour and it drives me mad. They have a room of toys yet choose to play bonkers games like this all the time instead. Have read loads of parenting books and have tried various strategies but nothing seems to work. What works with your dc, and do they play nicely together?