Comparing 2 Fermentations

Since I bought a 20 gal. brew pot I have been doing 10+ gallon batches and splitting them in half into 2 6-1/2 gal. carboys.I do a 2000ml starter with O2on a stirplate at least a full day maybe 2 days before starting with 1 vial of yeast.I have been noticing that one fermentation everytime seems to take much longer (bubbles in blowoff ) than the other! Everytime!!!!!! I think they ferment at maybe 1 degree different at the most as I have a foam box chamber and temperature control in 1 with 2 heater wraps plugged into that.I put O2 in both and split the whole starter in both going back and forth to get the same amount of top stuff and thick bottom yeast. Any thoughts of why I get full fermentation cycle with 1 in 5 days and the other is 2 weeks? I think the starter is BIG enought as the beers are only 1.055 +-.

My only thought is you are not evenly splitting your starter, why don't you try doing 2 equal starters as it seems you have 2 of everything else? It's the only unknown variable I can see.