I usually just carry one on me and one in my vehicle. The "truck gun" is always in there though except when I get it out to clean it and change ammo (About once a month).
I'm not sure about the high issue of that XD but one should be able to push that rear sight on an XD. I know people have changed sights on them so they have to be able to move. Most of them need a tool to do it right though.
Back to the first question I know people that carry more than one, I just haven't seen the need to personally.