FWIW, I've found IHG hotels to be unexpectedly generous with this. I've had maybe a dozen stays over the past couple years where I booked two rooms. As Diamond with Lounge Pass, I was granted club access and/or breakfast almost every time. This stands in contrast to Marriott, where one is usually asked to pay for the second room. Now, I'm reluctant to type this, because I'd hate for people to start demanding free breakfast for multiple rooms (which is the kind of behavior that has killed many a nice benefit over the years). So please keep in mind that the T&Cs stipulate breakfast for two people in the Diamond member's room, and anything beyond that is entirely up to the hotel's generosity.