The 6.0 product guide (page 910) says this:
"Note: Multi-Link is only supported with Stonesoft NGFW gateways at both ends. If an external
gateway device allows configuring multiple VPN tunnels between two devices, you might still be
able to use some Multi-Link features. Not all Multi-Link features are available with an external
But we've seen it to work with Cisco at least in active-standby link configuration. With active-active there is the question how the other end should select the tunnel to use, and AFAIK Cisco only has a standby option available (not sure about latest versions though). Aggregate mode for the links will for sure only work with between two NGFWs.