I totally know how you feel. I have a 3 year old girl Devon Rex cat and I thought she was lonely since I work all day...my brilliant idea was to go get her a little baby boy kitten (thinking that was safe since it wasn't another girl)....well it has been a year now and she still despises my other cat - Romeo, the now 1 year old...he wants to play and bites her, she wants nothing to do with him. She is also not going in the box and I have two boxes as well...
Long story short, I'm so sad...but I have to give Romeo, the little boy to my cousin. I just can't do this to Maya anymore...she is having crazy blood tests from stress from Romeo...the vet bills have been outrageous recently. It is so weird that another cat can have that sort of emotional affect on them.
So good luck! I hope you don't have to get rid of one like I am having to...it is so sad - but yeah I'd say after a year would be a good time to figure out if that is what you need to do or not.