Hi Blason_R,
ISP Load Sharing VPN works only in Check Point Gateways. For 3rd Party S2S VPN will only work with one external link, i mean that only one VPN ID (IP of Check Point Object) will share between Peers.
1) Example For Check Point Gateways Configuration: attached (name: CHKP ISP Load sharing VPN only)
In most cases for Check Point GW works with Load Sharing VPN, Route Based probing and IP address of chosen interface.
2) Example For 3rd Party VPN (Recommended): attached (name: 3rd party vpn)
Make sure that the external link for S2S VPN is used for IP Selection by remote peer.
3) To have any external link involved into a Site to Site VPN make sure that you have a Application Delivery Controller above the Check Point GW to balance any IP and DNS Query for Site to Site VPN and the global configuration by Gateway will be like this: attached (name: DNS Site to Site VPN)
So for any connection that is coming with alias (DNS Query) cp.abc.com will be negotiating.
Important: If you have any license of VSX (Virtual System Extension) you can create a Virtual Switch for each ISP and connect one Virtual Gateway to ISP1 and connect one Virtual Gateway to ISP2.
This feature is very important that we need a SD-WAN solution into Infinity Arquitecture to balance Default Gateway (2 and above) and NAT Outbound IP. That will help a lot for Site to Site VPN and Balance and VPN ID for any Link for Site to Site VPN, we hope that in the future releases we will have those features into R81.x.
Best regards,