The way i do is first install SP on Passive node(B) --> reboot it and then failover from active(A) to passive(B)
Now B is active and A is passive
Apply Service packs on A and then reboot it , then move the resources back to A.
For Active\Active
Failover to any one node eg A and then repeat the above processes and in the end move the resources back to respective nodes.
While changing Node1 from A to B change the owner likewise Node2 from B and then back.
Hope you are aware of the pausing part.