You're right. I was thinking of the similar-but-crucially-different situation of using two Apple Thunderbolt Displays chained together. Multiple sources confirm

You're right. I was thinking of the similar-but-crucially-different situation of using two Apple Thunderbolt Displays chained together. Multiple sources confirm that works, but obviously that doesn't help if you're trying to use a pair of non-Thunderbolt DisplayPort monitors. The combination of slightly lower resolution (1440 vs 1600) and the built-in controller seems to make all the difference.