best way to cluster 2 seperate F5 LTM in production?

Do LTM A and B have all of the necessary VLANs and self-IP addresses for handling traffic (for both their own original services and any services gained via consolidation)? If no, then you need to make sure that each device is configured with relevant VLANs and static self IP addresses. If the answer is yes, then I'd use SCF output to merge the A & B configurations onto a single device, add any needed floating self IP addresses, then create a cluster.

You will need two traffic groups, one set to prefer LTM A and a second set to prefer LTM B, perhaps even with Automatic Failback set.

If by templates you mean iApps, I'm not sure what the impact of application services would be on a merge operation, nor what impact a merge would have on the application services.