I use thunderbird because my work uses gmail. I guess (maybe) I could add the secondary work gmail account to my regular gmail. Not sure, the other alternative is to keep a separate incognito window or keep work email open in a separate browser.
You can log into multiple Google accounts at the same time. You then get a user switcher in the top left, which as far as I know does not invalidate existing sessions.
I just use Thunderbird instead.