Danaher says:

Your opponent has two legs – make sure you are capable of attacking both: Its natural in Jiu jitsu to favor one side for attacks. Most of us have a better rear strangle on one side than the other, most of us have a preferred side for the guillotine. In the case of the legs, they are so close together and it’s so common that an opponent can slip free from your ashi garami that you would be doing yourself a disservice if you could only attack one side well. Take the time to develop strong attacks on both of your opponents legs, because very often as you attack one leg and a good opponent escapes, his other leg will be ready to be attacked. The good news is that because your opponents legs are very close together, it takes very little movement skill to transfer form one to the other. Thus Learning to attack the legs on both sides is not so difficult to learn.