I've just been assigned my second supervisor, and will have my first meeting with her and my primary supervisor very soon. I was wondering what my expectations should be regarding the second supervisor's role, how often I should communicate with them, when it would be appropriate to defer to them etc.?

My second supervisor during my MA was in name only, as they were randomly assigned and were only meant to be contacted if our supervisor was unavailable/if we had a dispute.

Second supervisors vary greatly in their role and how much they help. Ideally, a second supervisor would take an active role in the progress of your PhD and have regular meetings. This is because if you have a dispute with your primary supervisor or they leave/get ill the other supervisor should be able to take over supervision with ease. My second supervisor only ever checked my progress reports, reports done every 9 months to continue with the PhD. I think it's best to ask them how much of a role they want to take. Them showing to a first meeting is a good sign, something my second supervisor never did.

That sounds like my second PhD supervisor! I think the second supervisor roles varies drastically depending on circumstances, nearly as much as the main supervisor role. It all depends how much you need them, how interested they are or if they are just a placeholder etc. There is no way to predict the relationship beforehand but you could try and set your expectations early in the process. Ie meet at least once every 1-3 months, they are going to help with certain things. The more involved they are at the start the better.

My second supervisor had no active role in my PhD as far as general supervision was concerned.
I published with him and he reviewed the papers that I had written but the nature of my work meant he didn't really have the ability to check the details.
Fortunately my relationship with my primary supervisor was very good throughout so I didn't need him for anything else.

My supervisor has done little to support my research....The only use of him is when my primary supervisor is away and I need a signature, he can do it on behalf of my primary supervisor...So don't expect too much from your second supervisor

Seems like the role of 2nd supervisor varies uni to uni, department to department, maybe even between individuals within a dept.

At Masters my second supervisor was just meant to be a secondary marker to ensure fairness, but ended up filling the role of my first sup when they went sick. They were really helpful and involved.

At PhD (so far) my second supervisor is second in name only really as both supervisors are active, interested and hands on, giving their different viewpoints. This is in the social sciences.

My primary and second supervisors actually did a 50:50 split on supervision. Might be rare but they were essentially part of the same group.