In order to not send email from the wrong account, I “Show in New Window” the second account. So I have 2 windows, each with focus on one email account. So far this has worked well in that I don’t start a new email from the wrong account and send it (i.e.,. accidentally send a work email from my personal email account).
In onder to use Show in New Window, I need to expand the account list, this menu option isn’t available if the account list isn’t expanded (to the right). Then I collapse the account list again to leave more screen space. I’ll submit a feature request, but it sure would be handy if I can use “Show in New Window” from the collapsed account list. That would make start-up so much more efficient.