This is partly how 2FA Electrum wallets work. When you set one up, you create a 2-of-3 wallet between yourself and TrustedCoin. Your wallet holds one key, and TrustedCoin holds another, allowing for 2FA spends. However, you can also restore this wallet and bypass the 2FA by entering a single seed phrase. This is because Electrum derives two sets of keys from this single phrase, one at m/0' and another at m/1', and imports them both in to your recovered wallet, allowing you to spend without the need for TrustedCoin's key.
I would note that doing this negates a large part of the benefit of a multi-sig wallet, in that the compromise of a single back up will compromise your entire wallet, rather than requiring the compromise of 2 or more back ups in a traditional multi-sig set up.
A slightly more complicated (but better) solution is as follows. If an attacker was to find one of your back ups as above (containing 1 seed phrase and all other xpubs), although they could not steal your coins, they could recover your addresses and be able to see how many coins you are holding. To avoid this, you can create a back up system which does not store every xpub on every back up, but still means that any two back ups (in a 2-of-3 wallet) are enough to fully recover your wallet. Your three back ups would look like this:
1 - Seed A, xpub B
2 - Seed B, xpub C
3 - Seed C, xpub A
If an attacker finds one of your back ups, they can learn nothing about your wallet or your coins. Any two back ups gives you two seed phrases and the third xpub in order to restore your coins. The same system can be expanded to cover 3-of-5 or other multi-sig combinations, if you choose.