I think I'm making a mistake in my thinking, if I use a MultiSig with Electrum Coldwallet + Specter Shield and I enter the seed from the Specter Shield manually into Electrum, then I don't need the shield at all, since I can sign with both seeds with Electrum
Exactly you don’t need the specter shield for signing because what is needed for signing the transaction is your private key which is already on the electrum wallet since you imported the seed phrase already. The only time you need a hardware wallet for signing is when you connect your electrum wallet to it and on that the seed phrase is not known by the electrum wallet.