Is it possible to select a bone underneath a bone?
Simple answer is yes.
For my first test I create an arm and a foreshortened arm. However, I unthinkingly created them such that the upper arms were aligned on top of each other. The implication is that when I created the bones, one upper arm was on top of another. That should have been my first clue, since I had to hide bones to do it. In any case, I then moved to creating the Vitruvian group and realized I couldn't select the bone underneath the top one.
Solution:
The bones are drawn as outlines, just select one bone then either ctrl-click or shift-click the other bones you want to group. Make sure you click on the bone outline, rather than clicking inside the bone. If you click inside, it will always select the one that's on top.
Discussion
In my initial attempt I seemed I didn't seem to be able do it. In retrospect, I was clicking on the inside of the bone, not on the bone outline itself. But, you might say, "My bones are very closely aligned and it's difficult for me to click on them independently." The base of the bone may be aligned, but there should be some part of the bone that is easily distinguishable. They shouldn't both/all be pointing in the same direction and have the same length. If they do, then that should be the base/common bone for your group (See the tutorial).