Join Date: Nov 2013
Location: Cape Cod, Massachusetts
Hissing on Shiro's part is entirely normal as right now he feels his territory is being invaded. Even though he might 'know' your new cat he has never been in the house before which seems different to Shiro. Two days is too soon to worry though - and also too soon to be introducing them face to face. Just leave the door closed to Ginger's room and let Shiro get more used to the idea of him being in there. You can feed Shiro near the door if he'll eat there, play with him near there, feed him treats near there - everything to create positive associations being near Ginger.
After 5 days to a week, I would get two baby gates and stack them in the doorway of Ginger's room so that he and Shiro can see each other. Continue the feeding in that area, the playing and treats. You can give both cats treats and play with them both, while they are still separated by the gates. You can also wipe the cats down with towels or cloths of some type, and then wipe Shiro down with the one with Ginger's scent and vice versa - or just let him smell it and be around it if he objects to having it on him. When Shiro's reactions become calmer, then is the time to try to introduce them briefly face to face. It can be a long process but definitely worth it in the long run.