Dont worry about BBS, I think this is where you are getting the irritation, a good two pass shave lasts me a day and a half and it sounds like we are basically the same if you are getting the shadow at day two.
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I have a very heavy beard & find that if I'm using a super sharp blade (Jnat finished or the like) I can shave every day w/ no problems. If I use something like a coticule or Norton edge (although perfectly shave ready & capable of giving a very good shave), I do better shaving every other day. For whatever reason, I'm much more likely to get ingrowns & even break out slightly when shaving too often w/ a less refined (but again, perfectly shave-ready) edge. Probably has to do w/ less pressure leading to less skin removal & less irritation.
Just go with what's best for you, I have no problem with skin problems even when I shave twice a day, maybe because I've lived most of my live in the outdoors because of my work and have tough skin.