I cannot work this out, and are my assumptions correct?
Multisig 2of 2 :
Trezor(say) with electrum (electrum must be online?)
Pros: mix of different vendors/software to prevent exit scam or fatal single vendor bug
Cons: electrum online
Vs
Electrum with electrum multisig, both cold storage
Pros: nothing is online
cons: single vendor
If you use Trezor with electrum you're using electrum's multisig, so both options are one an the same.
I'm pretty sure you can create an electrum multisig with multiple vendors for hardware wallets.