LACP Between Fortigate HA A-A Cluster 620B and 800C

Hi, 

I have 2 Clusters (one of 620B and one of 800C), i need to configure and LACP between them, whats the best way to do this:

Do I have to create different groups for masters and slaves?

Is there any l2 switch needed to be involved in this configuration, or can i create a full mesh whitout it?

and if needed, what kind of config does that switch will need?

Thanks for your help.