Well, the resulting score reflects how well (or badly) an answer is received by the community as a whole, so I think you shouldn't be bothered with it when making your decision on how / for which answer(s) to vote.
By upvoting only the answer with the lower score, I would feel like I'm really more concerned about how "well-liked" the answers look to other readers instead of just voting according to my own opinion.
Furthermore, I see upvotes as a means of showing appreciation to the person who went to the trouble of putting together what I perceive as a good answer. I don't see why I would show that appreciation only to the author with the lesser score on his answer.
Lastly, voting in a way that makes the scores equal (or closer in case the difference is bigger than 1) would mean that my voting behaviour is time-dependent.
Let's say answer A has 3 votes and answer B has 4. Now I would have to vote vor A. If I came an hour later, the scores could have changed to A = 5, B = 4 (two additional votes vor A, none for B). So now I vote for B instead? The content of the answers hasn't changed at all, why would I switch my voting?
Wouldn't I have to keep checking the scores? Maybe tomorrow the answer I chose to vote for has overtaken the other by a significant margin now so I need to take away my vote from that one and change it to the other?
If left a choice between two (or more) equally good answers, I would always upvote them all.