If multiple internet uplinks are available, the VPN tunnel works only with one of them. The idea now is to use all internet uplinks for one VPN tunnel. The intrasite connection between the two UTM's speeds up and if one ISP goes down you have a failover without an interrupt.
Not right - One VPN Tunnel per Line works and failover. But if you want aggregate all lines with one VPN this doesn't work.
Gabriele Minniti commented
Already implemented... Since v9, you have a chance to create interface groups, in order to group wan interfaces and use those groups in the ipsec connections configuration, as tunnel interfaces, instead of uplink interfaces object or single interfaces.