My understanding is that if you use Authy, without backups/multi-device options, none of your account keys are stored on their servers. It all resides on your single device.
Ie, even if Authy servers get hacked and all their data is compromised, and the attacker knows your gmail password, he will still not be able to access your gmail account because he doesn't know your "Google 2FA keys."
If you use the multi-device mode on Authy, is this still true?
Reading through their blog post (http://blog.authy.com/multi-device) and the functionality description, I suspect not. In order for your multiple devices to be synced with one another, their backend servers will need to know your gmail 2FA key, and redistribute it to all your devices as needed. This implies that if their servers get hacked and their data is compromised, your gmail account will be compromised as well.
Is my assumption accurate? If an attacker knows my gmail password, and is also able to hack into Authy's server/database, and I'm using the multi-device option, would the hacker then be able to access my gmail account?