[pierrebai]

Or multiple profiles.

For email, I used multiple instances of Mozilla Thunderbird with multiple profiles. The existence of the multi-profile capability is not well advertised and not user friendly, as I need to create desktop shortcut with command-line arguments. But it works: i now have two different office 365 ID and they don't step of each other's toes.

(Yes, they do need to be completely separate processes. Tab would not work. OTOH, I find it soothingly reassuring hat they are separate processes. I feel the risk of the program have a subtle bug and starting to mix the two ID much, much lower in this way. It would need to be quite a severe bug.)

I did set each Thunderbird profile to use a different theme so that I can try to avoid confusing them.