I have tried that with a key I have, but it does not let me?
there's more than one private key behind every address in a multisig wallet. there are 3 out of which you need any 2. if you had the seed you'd have no problem because that contains information for 2 private keys. as it is you don't have the seed. you have the wallet file which contains only one private key. the other one is held by the company that verifies the google auth code i.e. trusted coin. they won't sign your transactions until and unless you show knowledge of the google auth code. so you have neither the seed nor the google auth code. as a result your coins are gone. lost.
next time write down the seed.