Welcome to Purrs, sheff utd. When you say that Susie has stopped getting on with your other cat, can you let us know a bit more detail?
Also, what has prompted you to think that the imminent spring weather - and the renewed attractiveness of the outdoors to Susie - won't solve the problem?
Last, may I ask why it is Susie that you have decided to rehome? Is she the most recent arrival?
This may seem like a lot of questions, but in order to help you and Susie, we need these sorts of details