Not sure if it’s exactly what you are after; but I use the filter buttons in the middle panel to filter the projects and tasks for one or several areas.
It dosen’t however affect the left panel, it will still show all top level projects there. And it doesn’t affect what tag filter buttons are shown at the top.
However, I find it much easier to clich the area filter button than to use the Area dropdown menu.
Yes, that is how we envisioned multiple area selections to work. The top left area “switch” is really intended to be used as a way to focus on just one area. For example when you are at work and really don’t want to see any of your personal items in any views.