How to backup a secret encrypted by multi parties without revealing the secret?
If the goal is to store the secret on two servers and Alice so that, if Alice had access to both servers, you can recover the secret, but neither server by itself could (even if you could recover Alice's stored value).

If so, this problem is essentially secret sharing.  In this case, it can be done fairly easily:


Alice generates two random strings $P, Q$ as long as the secret $S$
Alice gives the value $P$ to server 1
Alice gives the value $Q$ to server 2
Alice stores the value $P \oplus Q \oplus S$ (that is, the strings $P$, $Q$ and $S$ exclusive-or'ed together) in her storage


It should be obvious that neither server (even with Alice's stored value) can recover the secret $S$; however if Alice can recover the secret if she has access to both.

And, secret sharing can do more than that (using somewhat more sophisticated mechanisms); Alice can share her secret among 5 servers, so that if she has access to any 3 (perhaps two of them crashed), then she could still recover the secret (but all the server's without Alice's stored value could).