Poor Shadow, I'm afraid I don't have any advice as both my cats were brother and sister so I've never had this problem. One of my cats passed away recently and I've thought about getting a friend for Bonnie but I don't think she would take it very well !!! I hope the 2 cats start to bond soon, I know how quickly you get attached to the new cat and it would be a shame to have to rehouse him !!!

Good Luck I'm sure with time they will come round !!! Fingers Crossed !!