You'd have to make 2 separate reservations - one for each resort/set of dates. Just remember with a split stay that your dining plan and park tickets don't carry over from the first to the next reservation so if you are getting free dining, you would need to book park tickets twice to qualify, and similarly even if you are paying for the
DDP. For this reason it may be worth booking the first leg of your trip with Disney UK with length of stay (or almost) park tickets and then going onto the US site for the second one and adding a one day base ticket to get the DDP on the second.
We've wanted to do a split stay so many times but due to the park tickets/dining plan challenge it has been a bit cost prohibitive for us