i've read a few horror stories about greenaddress's multisig and 2fa implementations---people being locked out of their wallets for 90 days, or permanently.
i would recommend against them, unless you are an advanced user or are very familiar with the UI. it has too much complexity and too many unnecessary features.
It is then something very serious and does not look like a wallet for beginners, it would be good to add some links as a warning for anyone who still decides to try that wallet. I am always in favor of greater security, but if this can lead to a complete loss of funds, I will still opt for some simpler solutions.
Electrum also has 2FA, but I don't think that option exists in the mobile version (maybe I'm wrong?). Also if you lose access to 2FA and have access to seed it is very easy to make a wallet without 2FA, and even if there is no seed Trusted Coin often came out to meet those who were in a very awkward situation.