2 breakers - should they work as one?
Assuming this is in the USA, 120V system, there are generally two possibilities:


240V circuit


A dryer, range, air conditioner or other large appliance will use a 240V circuit, which will be implemented in the breaker panel as a pair of breakers - one on each "leg". However, that is normally for 30A or more. I can't think of any reasons to bother with 240V that wouldn't also use more power. So 15A x 2 is much more likely to be...


MultiWire Branch Circuit MWBC


This uses two breakers, each controlling a single hot wire and sharing a neutral. The breakers need to be tied together for common shutoff. Because US wiring does not normally have breakers on the neutral wire, without common shutoff you could turn off the breaker on one circuit in order to work on it but if there is any usage of the second circuit then there would be current flowing through the shared neutral wire, which would be a major safety hazard.

Where are MWBCs?

They could be anywhere. A typical use is to have one hot power the top receptacle and the other hot power the bottom receptacle in a series of duplex receptacles. But really they could be anywhere (including receptacles, lights, etc.) and, from the perspective of the person using the receptacle (plugging in a toaster or radio or vacuum cleaner or whatever) it looks and functions no differently than an ordinary circuit.