Update: after a few additional days at 66, the Oktoberfest dropped a few points and is now at 1.016. Sample definitely tasted less sweet. Right now, assuming no additional fermentation, I'm at about 74% attenuation.
Doppelbock, on the other hand, appears to be stalled at about 1.027, which would be about 69% attenuation. That's a bit weak for this strain, which I don't completely understand, having pitched a healthy starter with about 90 seconds of oxygenation (wort was about 58-59 degrees when the yeast was pitched). Fortunately, I'm not getting an overly sweet taste from the samples.
I'm going to let it go until at least tomorrow evening to see if I can squeeze a few more points from the doppelbock.