I think this was possible in Blender 2.79 to easily select multiple hierarchies, but it changed in 2.80. I'll edit my post if i remember what it was.
In any case, ever since I work with Blender 2.8x i use a workaround.
It's not perfect but I for one am glad it is there.
Hierarchies in 2.8x
In Edit Mode in the 3D Viewport you can make a selection grow by Ctrl + Numpad +. But the same works for hierarchies.
If you select objects (possibly form multiple hierarchies), either in the Outliner or in 3D Viewport, then (with the mouse over the 3D Viewport) press Ctrl + Numpad + multiple times, everything in the hierarchies gets selected. The number of key hits depends on how deep the hierarchy branches/goes, so in some cases you might have to hit Numpad + a few times.
Collections in 2.82 (but apparently not in 2.83)
For collections right clicking in the Outliner does work to select multiple collections. You can right click on a collection in the Outliner and choose Select Objects. If you then click on another collection and do Select Objects, they are added to (not replaced by) the previous selection.
Hierarchies in 2.79
I just had a look and remembered; might be helpful to anyone working with 2.79.
Clicking on the root parent in the Outliner while holding Ctrl selects the whole hierarchy. Clicking on an object in the Outliner while holding Shift adds it to the selection. So holding both Ctrl and Shift down while clicking on the root parent objects quickly slects multiple hierarchies.