In some ways I think Chloe preferred to be an only cat, so I would consider the personality of your resident cat.
Iris tends to dominate games, so we try to make special times so Chloe gets a chance. (Chloe loves to watch and stalk, whereas Iris pounces every time and never gives Chloe an opportunity.)
I do feel better that they have each other's company, even if they sometimes get irritated at each other. Particularly if we're gone for much of a day. They chase each other around and get exercise and interactive play.
As far as supplies, double the litter, food, literboxes, check-ups at the vet, etc. Otherwise, it's really not that much more work.