I don't understand why you have to have internet to sign something, for example Jade doesn't need internet to sign either...
Theoretically it should work that you connect the Trezor with an airgap Electrum so that you can sign without internet or am I wrong?
You don’t need internet to sign transaction but you need internet to actually broadcast transactions and that’s why jade hardware wallet is connected to blockstream green wallet or the transaction data is shared using other methods like QR code or usb. The signing is done on the airgapped Jade wallet but broadcasted on this wallet. As for Trezor I didn’t say it wouldn’t work with Electrum airgapped wallet but there is no need for that your Trezor doesn’t share its private key with any connected wallet and that’s basically what airgapped wallet are also for so why having them again in the set up when you can just go straight and connect the Trezor wallet with an online wallet that will be used to broadcast the transaction after signing.
But if you’re going in for a maximum security then go just use the electrum wallet on airgapped device and create another watch only device for broadcasting of transactions, this setup doesn’t need a Trezor or any other hardware wallet between them.
But I would like to set up a multisig...
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You can simply setup a multisig with the three wallet, a 2-2 multi sig wallet will be ok (although I don’t like the idea I prefer a 2-3 multisig wallet). One signer will be on the Trezor hardware, one will be on the electrum airgapped wallet and then a watch only wallet for signing the transaction. The must important thing is to never have your consigning wallets generated or signed using same device