Lots of good advice already, but I wanted to throw this in there too...
My older horse is a terrible companion horse. Sure, she keeps my primary riding horse company when she's in there, but when I take my primary horse anywhere (including staying on the same property, riding in view of older horse) she freaks out. She can't stand being alone, so she either tears up her stall, or just jumps out the window, which caused her to injury herself. It's very annoying to deal with.
So make sure the companion is quiet and easy going, because otherwise it will make it difficult to ride your horse.
(my pony comes back soon to keep her company, thank goodness!)
In which I go on and on about horses.