I have two tightly coupled services that can run on completely different nodes (e.g. ServiceA and ServiceB). If I start up another replicated copy of both these services for backup purposes (ServiceA-2 and ServiceB-2), what would be the best way of setting up a fault tolerant distributed system such that on a fault in any of the tightly coupled services ServiceA or ServiceB the whole communication should go through backup ServiceA-2 and ServiceB-2?

Overall, all the communication should go either through both services or their backup replicas.

|---- Service A
|   | Service B
|
| (backup branch - used only on fault in Service A or B)
 ---- Service A-2
    | Service B-2

Note that in case that Service A goes down, data from Service B would be incorrect (and vice versa). Load balancing between the primary and backup branch is also not feasible.