The shared field of CenturyLink Field has generally been a success. Only once have any kind of markings been on the field. The 2013 Seattle Seahawks schedule leaves plenty of space for the Seattle Sounders. There are two dates which could cause doubt, both during the MLS Cup Playoffs. Getting ahead of ourselves, if Seattle wins the rights to host the MLS Cup Final there would be no lines as the Seahawks do not play on that weekend.
During the season proper there is only one shared weekend. On Friday September 13th the Sounders play. Later that weekend the Seahawks will be the Sunday night game on NBC facing the 49ers. There is no other tight overlap. The field will be clean.
The questions about the Playoffs come about because MLS has only released the date of the MLS Cup Final (December 7th or 8th). With the other dates in doubt the full month of November must be considered. The Seahawks play twice at home that month. November 3rd they host the Buccaneers. On November 17th they face off with the Vikings. There are a multitude of ways that the MLS Playoff schedule could be worked out, especially with an entire extra week compared to last year. December 2nd the Seahawks host the Saints, but the week prior to MLS Cup was an off week last year and that seems likely to return.