I believe that according to announced attendance, the Sounders have outdrawn the Mariners every time both teams have played at home on the same date this year. Today, playing at night while the M’s went early, they got within shouting distance of doubling the M’s total.
4/17, Mariners 31,647 to Sounders 35,924
5/1, Mariners 30,225 to Sounders 36,219
5/8, Mariners 30,446 to Sounders 36,273
5/26, Mariners 22,422 to Sounders 40,122
I don’t know what MLS’ standard for sales is, but MLB uses tickets sold and not butts in seats.