Not sure what you are asking. Normally when people ask for 1/2 and 1/2 on a sheetcake they mean like 1/4 sheetcake in vanilla and a 1/4 sheetcake in chocolate and then you lay them side by side to make a 1/2 sheetcake. This is so the guests can have a choice of flavors.
I'm the same as renee. You dont' need a barrier. It's not Kool-Aid, it won't all run together. It's batter .... it's like lava ... when the two batters hit each other, they will stop.
I turn a saucer upside down and place one end of the 11x15 or 12x18 pan on it. Then pour one batter in the low end. Remove the saucer and pour the other batter in the other end. Bang the pan and throw it in the oven. Way easier than baking two cakes.
People actually fight over the center where it's 1/2-n-1/2! 
Same as Indydebi except I just use the edge of another cake pan to rest one side on. It doesn't need to be a PERFECT dividing line. Just make sure the two batters bake the same...for instance, I won't bake a 1/2 white and 1/2 carrot cake together.
I've done this sooo many times. I cut a piece of sturdy cardboard to the width and height of the cakepan. Then cover it in plastic wrap and place it in the middle. If you cut it just slightly wider than the pan, it will fit tight. Pour one side, then the other, and shake to level. Then take out the cardboard and it's a near perfect division.
When I do this, everyone always fights over the middle too.
I've even done this with three flavors...vanilla, chocolate, and strawberry. Good luck! HTH
Hi, I do this a lot too and I just use a cake board circle. I have one cut in half for each size of cake pans I use and when its time I just use it over and over, but.....I cover it with foil each time to keep the cardboard circle clean. Just cut them in half, you will need different sizes for different size cakes too. Sandy
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