I use a piece of cake board that I cut to fit from side-to-side in the middle of the pan or you can fold a piece of aluminum foil and stand in the center of the pan ... I mix and pour one side, then mix and pour the other side; example; half white, half chocolate.... after you've poured both sides, I slide the foil or cardboard out from the center then bake.... they cake batters will remain separated .. I've also heard where some people have even tilted their pans so that the batter stays on one side until they've poured the other side of their pans .. I just prefer to do a cardboard or the foil .. hope this helps 
I did this for the book cake in my photos...half yellow and half chocolate... what I did was mix up my batters, then 2 of us started to slowly pour the batter in each side..and it met in the middle perfect.... if I was doing it alone, I would probably make a divider of some kind....
happy baking,
cakegal
The divider sounds great! I usually just pour both at thesame time. WHEN I had just my one mixer, I would pour the batter into a rimmed bowl and mix my next batch, then with one bowl in each hand ... pour. I've done this with a 12 X 18 and just yesterday in a 8" square.
I must add though, as far as cake mixes go, be aware of the types of batter. Some batters require more cooking time than others so lower your temp, cook longer and meet the time somewhere in the middle.
Rose
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