Temperance and the Lovers, as I think I mentioned In another thread are not quite the same.
Think of the Lovers as an apple that's been cut in two. The two halves find each other and reunite, the apple is whole again. Thus, with the lovers, the "two-becoming-one" is not really two into one, but rather half-and-half finding each other. You find your soul mate and it's like you're finally whole, like you've never been complete without them. So it's "half-and-half becoming one."
Temperance, however, is two different elements--maybe even elements that seem to be opposite each other (thesis and anti-thesis). Combine them, and you get a completely different third element--unlike either of the two other parts. You get, if you will a "child" or synthesis. This is "two-becoming a third" as it were.
Justice is doesn't try to unite two things, it tries to balance two things. In fact, if you have one thing, like a pile of money, the job of Justice is to SPLIT the money into two equal halves so that two parties can each get their fair share. Justice doesn't care if those two parties ever unite or like each other or become friends...just that the scales are balanced and justice is served. This is "One being split into two to achieve balance and fairness."
Actually, it creates an interesting story of a marriage. Two find each other, marry (lovers), have a kid (Temperance) then divorce and split things evenly (Justice).