I only setup ONE wallet with the Trezor after initializing it.  Why are there two Master Public Keys?

In Electrum, it says I have "Account 1"  and "Main Account."

I want to perform offline signing with the Trezor on an airgapped computer, with a watch-only Electrum for broadcasting, and would like to know which master public key to import for the watch-only side and why.

Thanks in advance.