I ended up boiling the first decoction for 20 minutes and the second for 30 minutes. Hit all my numbers so far (knock on wood). I'm currently about halfway through a 90 minute boil.
I'll be brewing a dunkel in a few weeks. Thinking I'll boil the first decoction for 20 minutes again, but I'll boil the second one for 40 minutes. I just brewed this dunkel a few weeks ago with a single infusion mash. I'll be doing some side by side tastings when the double decocted one is ready.
I did a triple decoction Oktoberfest this year and boiled all for 30 minutes. It came out great.