You can definitely have two profiles open simultaneously - on MacOS, this is achieved with “open -n Firefox.app --args -P [profile name]”; on Windows, “firefox -no-remote -P [profile name]”. Admittedly, that should _really_ be exposed in the UI somehow, but at least it means you can wrap up the command in a handy desktop shortcut (e.g. one that says “my Firefox” and one that says “her Firefox”).