No, I don't think you're doing anything terribly wrong. Mine tend to take at least 2 hours per pound too. I usually estimate 2.0-2.5 when I do mine and I have never had a bad one.
When smoking two butts, you could bump the time to 235. But in general I like to smoke at 225, no matter how full of a load I have.
2.5-3.0 maybe seems a tad long. But not totally out of the ball park.
1. Are you using a permanent probe or the standard?
2. Where was the smoker probe located? It could be a bad probe placement that was causing false readings. I used to always put my probe on the underside of my smoking rack. But have since started moving my side rails to the top position and placing my smoker probe on that very top rack. I have been getting better readings from this location.
Did you brine your butts? If so, brining does increase the time a bit, but well worth it. I ALWAYS brine my butts.
Did you have a water pan of any kind in the smoker? If so, where was it located? If it was anywhere other than the bottom of the smoker resting up against the smoker box, that could be causing problems as well.