One edge of each race is thin, the other is thick. The thick side is the load-bearing side of the race. You will notice that the thick side of the inner and outer races of one bearing oppose each other.
Of course, you always want to orient the races so that you have "thick facing thick" or "thin facing thin" in order to obtain any kind of factory preload on the bearing pair.
As to how you should pair them, it really depends on how you plan on retaining them in the housing or on the shaft. If the retainer is a nut on the shaft, then you want the outer races to be "captive" to the inner races, so this means that you orient the pair with the thick side of the inner races facing out, one against the shaft shoulder, and the other against the nut.
If the retainer is a nut in the housing, pressing the outer races together, then the opposite orientation would apply.