ATP and Ariben Aviator are specialists on doing the IR on a twin. They use the Seminole and Duchess respectively. Both schools have their problems but are okay overall. Expect to pay about a hundred quid an hour. I'm sure other schools would do the IR on a twin on request, but what experience do they and their instructors have of this approach! You will need to do an engine out approach for the checkride. Don't be surprised if it's on an NDB with a partial panel. And yep, I did my IR on a twin and it worked out very well. It was still tougher than doing it in a single though.