Yes, you can cluster two boxes. However, I believe you need an external load balancer for Active/Active. Active/Passive, no load balancer. I hope this helps.
yes, clustering for the MAG is the same as the SA series; you can do this with one or more chassis units (as needed/desired). this includes the caveats as noted in the admin guide (e.g the need for a load balancer with an active/active cluster)
How is MAG HA supported if you have 2 sites (site 1: MAG6611 with 2 SM360 A/A and site 2: Mag6611 with 2 SM360 A/A)?
In other words if one site goes down how does the 2nd site pickup the traffic?
Global server load balancer + local load balancer?