Fermenting separately, potentia problems?

Made a batch that was too large to fit into one carboy, so I put 3.5 gallons in one carboy, and 3 in another. Pitched a starter of Wyeast 1335, and fermentation started strong after 12 hours. Continued strong for 48 hours with both carboys fermenting at the same rate. After 48 hours the krausen dropped on the one carboy, but not the other. It's been 6 days, and the one carboy still has 4 inches of krausen, still fermenting, and the other has been pretty much done for 4 days with only a few bubbles.

What's the deal? Did I maybe get more yeast in one than the other? They've been at the same temp the entire time. I'm planning on waiting till the one is done, then checking the gravity of both. What would make one would take so much longer than the other?

Depending on how you split the batch, you could have more sugars in one fermenter (did you take separate OG readings?). Or, as you said, more yeast.

I doubt it will be a problem. Krausen and bubble rates are not the best indication of fermentation activity. Check your FGs according to plan. Even if they're a point or two apart, all should be OK.

I actually just checked the gravity last night on both (9 days in primary) and the slow fermenting one is at 1.024, and the one that finished in two days is at 1.020. My target was 1.018, but I mashed a little high (156-158) so I was expecting it to be on the high side.

The slow fermenting carboy is still spitting out a bubble every minute or so, so i'm going to just leave them both for another week maybe.