Performing two arm rows allows you to train the back in less time. However, the one-arm row allows you to often train with heavier loads, train closer to failure, and not be limited by hamstrings and lower back positional strength (as you can use your other arm as a support structure).
If you find you do rows but do not feel the back muscles fatiguing out first (not the lower back, but the middle and upper back muscle fibers), try one-arm rows in a supported position. If you want to do two-arm rows, try performing the chest supported two-arm row option from above.