This is an older post but I thought I might mention:
Right click on the tab of the schematic or PCB window panel and the context sensitive menu has an option to split vertical (or horizontal). To rejoin them use the merge all selection.
Also, you can detach a tab and drag over to another monitor by left click and drag the tab. Then you can have two windows open at the same time to see multiple schematic pages and for schematic and PCB viewing. Drag the tab back to the other monitor to redock to a single monitor. You can also do this with the Open In New Window selection on the right mouse click of the tab. Merge all is then grayed out so just drag the tab back to the main window to merge back.
Also, it is worth mentioning that docked work panels can be tiled or overlayed with access tabs. To move them grab the work panel title bar with the mouse and drag it. If you grab it near the actual text of the title it will separate overlayed panels. If you grab more toward the center it will keep the overlayed panels joined but float the entire work panel. Basically, there are hidden hot spots in the work panel title bars.
I mention this because it isn't very obvious at first and I ended up with tiled work panels and it took some time to figure out how to get them back into tabbed mode.