How to backup a secret encrypted by multi parties without revealing the secret? I'm searching for a way to backup a secret that only the end user knows the information with 2 trusted hosts.

But trusted hosts shouldn't have access to secret at any time.

First basic version : 
Alice encrypt secret using Server A public key
Alice encrypt encrypted secret using Server B public key.
Alice store this double encrypted secret somewhere safe

To restore :
Alice ask server B to decrypt double encrypted secret
Alice ask server A to decrypt result
Alice get secret ... but server A also !

It could work if : 
to restore : 
Alice ask server B to decrypt double encrypted secret
Alice encrypt result using a magical encryption mechanism (?)
Alice ask server A to decrypt this information (homomorphic ??)
Alice receive result and decrypt using a magical decryption mechanism (?)
Alice get secret ... and server A got just an encrypted information !

Is an encryption mechanism like this exists ? 

Is there an alternative approach to my goal ? 

Thanks
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).