I don't know about two ravens but Twa Corbies is a Child's ballad ( #26 )which dates at least to 1803. He also has listed in the Student edition an older version called the three ravens which dates to 1611. Twa cabies starts, "As I was walking all alane I heard twa corbies making a mane." The Three Ravens starts, " there were three rauens on a tree, downe a downe, hay down, hay downe". I don't know if this helps or no but I thought you might like the info none the less