I think we know who would win if Hikki and Ayumi were to battle it out again with albums released the same day. Here are their first weeks (I included the Ayumi album closest in release date to Hikki's albums):

First Love - 2,026,870 - - - - A Song for XX - 548,210
Distance - 3,002,720 - - - - - Duty - 1,682,760
DEEP RIVER - 2,350,170 - - - Rainbow - 1,016,482
ULTRA BLUE - 500,317 - - - - Secret - 386,280
HEART STATION - 480,081 - - Guilty - 202,000