Stacking two panels works as well (sort of), but the inner panel will be covered by maximized windows (only panels at edges of the screen set struts (margins) for the window manager).

But then I suppose he could set a bottom margin in Settings>Workspaces.

An alternative (a new feature in 4.10) is to set the number of rows to 2 in the panel settings dialog. The panel will *not* look like the one on your screenshot but its functionality will be similar.

Stacking two panels works as well (sort of), but the inner panel will be covered by maximized windows (only panels at edges of the screen set struts (margins) for the window manager).

I used on LMDE (Linux Mint Debian Edition 1204) XFCE 4.8 the horizontal separator, to separate one row for my lunchers and one for my running applications.
Here is my results:

As you can see - lunchers | horizontal separator | applications (first row lunchers, second applications; the black box is just censoring)

I am setting up my notebook with Linux Mint 13 (maya based on ubuntu) and there is XFCE in ver 4.10.
There is no separator with attribute "new line" but I can set up the panel "2 rows". When I do, I have two rows for applications and my lunchers are all on the right (not in the first row).

Do you know how to reach the same result as I was before (with XFCE 4.8 - you can see on the picture)?
Thx for info