As a workaround for this one:
Allow selection of objects in separate groups from Title Explorer – can currently only select objects in different groups via the canvas
You can select multiple objects in separate groups by freehand dragging a rectangle around the wanted objects in the stage-view. Offcourse this is limited, but sometimes its a workaround when selecting objects in separate groups.
Doing it like this, you select all elements which bounding box falls completely within your drag-selection.