Come up with the formula (how many teams get promoted/relegated, how many teams in each division, schedules, etc), and then at the end of the season, the top 14-16 clubs (I don't think we're anywhere close to the end of the wave of expansion in the MLS) will find themselves in the newly formed MLS 1 division. Everyone 16 or below is in the MLS 2 division, with the prospect of being promoted at the end of the next season. The split would make for some SERIOUSLY good entertainment in the Lamar Hunt US Open. Granted, the clubs finishing 16th-last won't be too happy with the newly formed divisions, but it's got to begin somewhere right? (if it ever begins...)
But I agree, I don't think they would necessarily need 3 promotions and 3 relegated teams like the Barclays, Liga BBVA, Serie A, and Bundesliga does. One or two would be pretty ideal. At least to start with.