All the tile says is that it is Electrum 2.6.4
2.6.4?? That is quite old... is that the version on Linux or Mac?
HCP:
I recall setting up the co-sig as the Mac Wallet with MPK, I either sent a test amount to myself from Linux to Mac, or it was to an exchange (I can't remember) but upon opening the Mac Wallet (which leads me to believe that first test was to myself and I went into the wallet on the Mac to see), I was immediately asked to sign the transaction, it just popped up to do.
That initial test did not require the save transaction process, and loading in the other wallet.
I suspect that you had the the "co-signer pool" plugin loaded and running... this plugin sends the transaction to a central Electrum server waiting for the co-signer to open their wallet and automagically sends them the transaction to sign.
To be honest, I'm still struggling with the idea of using a MPK from a 2FA wallet as a co-signer for a completely different MultiSig wallet... when you setup the Linux wallet, was that setup as 2FA as well? Or just as a "normal" MultiSig wallet?