first hold down shift and select the layers you want to merge. then from the drop down menu on the layers palette select group. then go back to this menu and select collapse and it will merge the layers.
(this is painter 9 but its probably the same in 11?)
I was wondering the same thing. However, neither of the methods above worked. I drew on the canvas; created a watercolor layer; another watercolor layer above that; shift selected both but "collapse" was greyed out. Then tried a layer with oils and a watercolor layer; shift selected them; grouped them but "collapse" was still greyed out. Using Painter X.1 on MacPro.
You have to "dry" watercolor layers before collapsing. You can find the "Dry" command in the layers palette menu and also on the Painter menu bar in "Layers". Also, a shortcut for collapsing or merging layers is Ctrl/Command>Shift>X. In Painter 11, it's Ctrl/Command>E, same as Photoshop's