You would use the same frequency for all. They usually have a set of frequencies to choose from on a little switch, so if your neighbour happens to have the same thing you can choose a different frequency. Otherwise you might end up viewing his late-night porno channel
Just make sure you buy the same make/model, and make sure you get the seller to agree to take them back if this doesn't all work
... I don't want to get the blame!
Dave