California recently changed its election system, making it very similar to what this measure would do in Oregon. We should learn from our neighbor's mistake, not jump off the bridge along with them!
A single-ballot ("blanket") primary makes the primary election more important, but it will not increase turnout. California's primary turnout was 33% in 2012 (before adopting a blanket primary) and then fell to 31% in 2012, and fell again to just 25% in 2014. The blanket primary has reduced turnout in California! Turnout in Oregon's 2012 primary was less than 40%, but over 80% in the general election. Are you sure you want to increase the importance of a low-turnout election?
Having only the top two candidates advance to the general election will entrench the two major parties forever. If you love Republicans and Democrats, fine, but if you aren't satisfied with the government you have and wish someone different would be elected someday, this measure would destroy any chance of that happening.
Minor party candidates who promise genuine change — and whose mere presence in the general election is a powerful influence on the major party candidates — would be wiped off of the general election ballot in all but the most lopsided districts.
The "top two" feature also causes clearly wrong outcomes. In 2012, in California's 31st Congressional District, where Democrats enjoyed a 15% voter registration advantage, two Republican candidates and no Democrats advanced to the general election. How? Because the primary had four Democrat candidates who split the vote. A Republican was elected in a +15% Democrat district!
This measure takes the power to nominate away from political parties, making it impossible for them to prevent vote-splitting at the primary, and simultaneously creating a perverse incentive for other parties to run "clone" or "ringer" candidates to split the vote. If you thought politics was dirty now, this measure would make it much worse.
Don't repeat California's mistake in Oregon!