Sigh.. it don't work like that. They are colleagues, not mortal enemies that have to fight to the death with every product.
When Sony were making laptops, it ran windows.
Bloodborne runs on phyregine and that uses havok, which MS owns. There's a ton of Sony movies on xbl, etc. messy stuff, but just because they beef it out in this industry don't mean they can't share common ground elsewhere.
That being said, it's just certain things for in house companies only. Why? Cause it'll be harmful it the game they made games that ran bad under MS' watch or embarrassing if they made better games with it.
It'll be interesting to see what happens to minecraft 2.