This is the first time I've done that. In December I bought a pair of Nike Air Monarchs (my first Nikes in years, in fact) and they broke in well and turned out to fit me well, in this job that requires lots of walking.
On Saturday, finally, I shopped at a place where I could get another pair of them. Both pairs are black, as the job requires black footwear. I've been breaking in the new ones, and once I feel I've done so, I'll be alternating. Two pairs you alternate tend to last longer than when you wear a pair until they wear out then you replace 'em.
Years ago, I heard a factoid that I hope is true, that alternating between three pairs will last as long as five pairs you buy in a row, but I don't feel the need to get a third pair of work shoes. I do, however, miss having boots. I had boots until last year; I wore those out so thoroughly that parts of both heels had worn to the padding, so by the end I was basically walking on sponges. I am hard on shoes.