I have a Bitcoin wallet for android and I would like to know what happens if I back up the wallet and restore it in other phone. Will I be able to use the bitcoins in both devices? Thanks in advance
Yes you will be able to access the same coins with both devices. You can think about it like if you make a copy of a key to your home, and give it to someone; both people will be able to open the same door.
Bitcoin Wallet uses a deterministic key derivation, so you are able to use both copies in parallel and see the same balances in either.