I would like to know how safe is the Airgap wallet if it is used on 2 devices.
When you say 2 devices, I am assuming you mean one device with the full wallet which is permanently airgapped, and one device with only a watch only wallet with no private keys which is connected to the internet. Such a set up is very safe, and if done correctly, one of the best ways to store your coins.
Also can you tell me when you have to sign the transaction with the qr code is there any way the app can display the address you want to send but in fact the app is sending to another address?
If your live wallet does send a different address via the QR code, then you will be able to see that on your airgapped wallet before signing the transaction. As long as you always double check things and don't just blindly sign everything, then your coins will remain safe.
Assuming that your airgapped pc was infected already with a malware
Whatever device you are using as your airgapped device should first be airgapped and then completely formatted and have a clean OS installed on it. That way you can be sure there is no pre-existing malware.