What do you put in as the subject marker if there is more than one subject? For example
Adam と Sarah ___ すしがすきです.
The particle will be the same regardless of number, so you would use は. Whether it's one person or two hundred people who like sushi, the particle will not change.
Just the same as if there was only one person:
The topic of the conversation, marked by は, is the pair of both Adam and Sarah. Topics can be very complex phrases; they don't have to be just single words.