If you need 2 tunnel for same ip, it works only with policy based VPN
You can map ip to FQDN
set system static-host-mapping host-name foo1.local inet 5.5.5.5
set system static-host-mapping host-name foo2.local inet 5.5.5.5
set vpn ipsec site-to-site peer foo1.local xxxxx
set vpn ipsec site-to-site peer foo2.local xxxxx
Some new features will be with XFRM interfaces ref Route-based VPN :: strongSwan Documentation