Nearly everything is possible.....
But that capability is not within the node itself - however that does not mean you cannot achieve what you want.
So what you want is to have two external VXML servers for resilience - correct?
One way to achieve this is to a content switch - so from CVP Call Studio point of view there is only 1 address.
That would be the simplest approach - but requires a content switch / load balancer.
Another simply approach would be to script the Call Studio to route to "Subdialog - Server A" or "Subdialog - Server B" element.
And you could decide which one you route to based on a variable, decision element or round robin etc, and have a manual failover if a server is down.
I am not sure if you can catch an error if a subdialogs fails or not - so you could auto route to the other side.
You could test.
Content switch is the best approach I believe. Offers full resilience and allows you to take a server off line without impacting callers etc.
Gerry