I need to do a sheet cake that is either 12x18 or 11x15 and needs to be half chocolate and half vanilla. Is there any trick to make both flavors in the same pan or do I have to bake each separately? Thanks.

nope, you can either sit one edge of the pan up on an empty cake mix box and pour the batter of one flavor on the side that's down, then mix your next flavor and add to the other side as you slowly remove the cake mix box from underneath......or you can fold up a piece of aluminum foil and put it down the center of the cake pan and then add both flavors of batter to the pan and then pull the foil piece straight up and out of the cake batter allowing the two flavors to meet in the middle (but still keeping each side separate flavors)...........or you can mix the 2 flavors in separate bowls and have someone help you (or if you're really strong and really good) you can just pour the two flavors, one on each side simultaneously at the same speed and they will fill each side easily and stay separate.

I personally elevate the one side, no mess, no hassle, and it's quick!

if you're doing an 11x15 you'll need one mix of each flavor...if you're doing a 12x18, that normally takes 3 cake mixes so what I do is make 2 cake mixes of the vanilla and pour 1 1/2 parts of the batter in the one side, then add my chocolate cake mix and a box of chocolate pudding and everything else called for on the box to the vanilla cake batter and mix that up........then you get 1 1/2 batches of chocolate cake batter. I use devil's food choc. for my choc. cakes and it still ends up very dark and chocolatey because of the pudding mix.

I usually buy a pack of cake boards in 19" X 14" size, then end up trimming them depending on the size cake I need to put on the board. I then save the scraps of cardboard for when I do 1/2 and 1/2 cakes...I wedge the piece of cardboard right in the middle of the large cake pan and it remains upright so I can pour the mixes on each side, one at a time. Once I pour equal amounts of batter on each side, I remove the cardboard and I have a perfectly divided cake.

Needed to calculate the volume I needed so that info help.

Thanks for the tips.

I baked a 1/2 choc and 1/2 yellow 12x18 for my finding nemo cake. I baked one whole layer yellow, baked the entire other layer choc. Then I leveled each. You have to make sure that you level each layer on the same surface and at the exact same height. Then you cut each layer in half crosswise and "flip" the two like halves together. You will then have a 12x18 that is half yellow and half choc. Does this make sense? Hope this helps. I thought the idea of trying to pour 2 different batters in one pan would be a disaster for me. This technique was taught to me by my Wilton instructor and was very easy. Hope it helps.

Of course, I just realized, that my tip won't help you if you only want a 1 layer cake lol!

what KHalstead said .... with one small difference.

I use 1 choc and 2 white for a 12x18. Chocolate cake rises higher and spreads further than white cake, so I end up with a cake that is practically 1/2-n-1/2, even tho' the batter I used was 1/3-n-2/3.

Do you have any problems with one cake batter cooking faster than the other? I wanted to try a half yellow half chocolate in a 11 x 15 pan. I use to different type of batter. 

Do you have any problems with one cake batter cooking faster than the other? I wanted to try a half yellow half chocolate in a 11 x 15 pan. I use to different type of batter.