Ok, I am going to throw this out there and I could be 100% wrong, but I saw customers do it this way and it does work. So, just for some context, if you were doing this on Fortigate OR Cisco, you could set up brand new vpn tunnel WITHOUT always having to use same encryption domain, but sadly, in Check Point, thats not possible, as you have to define it on gateway object itself. Personally, I dont even think thats needed, as you could leave it by default (all IP addresses behind the firewall). In reality, what controls the traffic are the rules, not the encryption domain for VPN. So technically, that way, you could have same gateway used as backup, just would need a different rule to control the traffic within that separate vpn community. There is an option for backup gateway in global properties and object itself,BUT, thats only for remote access VPN...NOT site to site VPN.
I honestly cant think of any other logical way to do this without changing the type of VPN. Anyway, Im happy to do remote session and see if I can help you. Message me privately and we can set something up.
Cheers!
Andy