IMO it should be done like this. You have a hot wallet, probably on your phone that has a half key. You keep a QR code somewhere else with the other half. Combine these two and you have the cold storage private key. Additionally you could have the full private key in a safe or whatever that you never touch.

In this system you can top up your hot wallet by specifying all transaction details and then scanning the QR with the half code to sign the transaction and send it off.

This would roughly as secure as some sort of 2FA method. The attacker needs to have both the phone compromised, and the paper with the half key compromised.