The 55xx and the N2xxx cannot be stacked together, other than that you can treat both stacks similar to as if they are just one single switch. Stacking them basically makes them a single switch, so you would just connect them in any of the same ways you would connect a single 55xx with a single N2xxx.  If using a LAG however, it is best practice to split cable connections across multiple stack members.  So for example if you have a stack with three members and you have a LAG using two cables, use one port from two members so that your LAG spans at least two of the members.  Doing this for both stacks creates a more resilient network, so if one member goes down the link will stay up.  In other words, if you use two ports on the same stack member and that member goes down, then you've lost the connection, so it's best to split the LAG onto multiple stack members.

Hope this helps.

-B