you should only present LUNs to multiple hosts if this some sort of cluster (ESX, Microsoft Failover Clustering ..etc)
I got several sgs that I want to be able to share luns with. but when I add the host into another sg, I got a warning saying  The following hosts are already connected to a different storage group:  Connecting these hosts to the current storage group will cause them to be disconnected from the other storage group. Once disconnected these hosts will no longer be able to access LUNs assigned to the other storage group.   This operation will add the following hosts to this storage group:  Each host will be able to write to all LUNs within the storage group.
The following hosts are already connected to a different storage group