Getting toddler to give up dummy

My 2yr old uses a dummy (only to go to sleep with) but was wondering how to wean her off it when I've got 4mths old who also uses a dummy. 

Obviously if there was going to be no dummys in the house it wouldn't be a problem but when my other daughter has hers she is going to want to use it. I'm sure there'll be plenty of people dealt with this so any suggestions would help, thanks