Hi,
I didn't quite understand the scenario. Do you required a NAT IP for a VIP running between 2 load balancers or a VIP to be running between 2 SRXs?
The most common and open standard protocol for running a VIP is VRRP, which requires a switching infrastructure between the 2 physical interfaces for exchange of VRRP packets.
SRXs can also be operated in clusters for high availability.
Cheers,
Ashvin