When did you split into two batches? Did you split the grain into two mashes or the wort into two kettles?
I assume the first, since the second would have to have the same OG. The first would have two different OGs because you're two systems have two different efficiencies. Things like dead volume, sparge volume, mash thickness, mash time and mash temperatures can all affect the efficiency from an otherwise identical bag of crushed grain. That is assuming you split grain from the same crush, otherwise, crush tends to be the overwhelming variable.