I made a 1/2 and 1/2 quarter sheet cake last week. The last one I made, I had trouble with the chocolate end sinking after it came out of the oven. So this time I thought I had it. I swirled the area where the batters met in the pan and even put a flower nail in the choc side. Baked at 325-over an hr. The sides were all pulled away from the pan-the white was starting to get overdone and the chocolate STILL sank significantly while cooling. So, I am interested in how others make these cakes WITHOUT putting the batters in the same pan.....Thanks!!
Bake one of each flavor, cut each one in half and stack on top of the other piece to make a double layer sheet cake.
If you mean "marble" cakes, where there is yellow or white batter and chocolate batter in one pan...
I GAVE THEM UP.
I hate them. Hate, hate, hate.
I spent one night til the wee hours and made 3 separate 15" layers of marble cake trying to get one decent one to turn out for a wedding, and I ended up having to splice two together by cutting out the low sunken spots and filling them in with higher spots from another layer that also sank in spots. I will not do another one!
Box mix, different story--those work out fine, but scratch marble cakes are off my list forever. It's not just you...it's THEM, lol.
If you must make one, do a box mix. That's all I know. 
98% of my cakes are half and half. I use Pillsbury devils food and Pilsbury classic white. I put them in the same pan. I use an 11x15 pan which is perfect for two cake mixes. I also use baking strips. My oven is electric and just a conventional oven. I never have trouble. I wrote a tutorial on how to make half and half cakes back in June; but it hasn't been posted yet. Sorry you have such trouble with them!
Thanks for all your quick responses. I will do the 2 cakes and put them side by side-they just want a single layer. The mix I used was Duncan Hines and am tempted to try it with another brand, as I have a friend who also does cakes-doesn't use DH and doesn't have trouble with the same mixes in one pan. And I never used to....... 
I always bake the two cakes seperately and put together, although I do not split a quarter sheet, then you are stuck with eighth sheets. I only split half sheets and larger. I usually put a long thick bead of icing inbetween the two and push together, then scrape off what oozes out. If one cake happens to be higher I just trim it first to make sure they look level, before I glue them together. I don't have a problem with them sinking or seperating doing it this way.
I worked in a commercial bakery and we never split 1/4 slabs either 1/8 are a pain to deal with...but for the half slabs and bigger I would just put two different 1/4 slabs side by side and ice over them. You can hide anything with icing...so even if the two seperate cakes weren't exactly level no one could tell. Worked like a charm.
I'll never do the two in one pan again either. Mine are scratch cakes and the white was done so much earlier than the chocolate. you have to keep it in til the chocolate is done, but I was so worried that the white would be extremely dry from being over baked.
I had to be out of town right after the cake was done, so I couldn't start over, which is what I would normally do if I was that worried about it. I decided all I could do was deliver, check on the feedback afterward, and if there was any concern, I was prepared to give her another cake at no charge to make up for it.
the customer said it was great, but I was stressed to the max with worry. I've put two together before and will always do it that way in the future.
My best guess would be to bake the two separately, but making the effort to overfill the pan for the one that you're having trouble with it sinking, or even both. After they are baked and completely cool, go back with a leveler, and level them side by side on your cakeboard so you have a uniform height across. I don't have the large wilton leveler, only the small one. So for a large cake, I start from each of the four corners and bring it in as much as I can, then use a bread knife to finish the middle.
I did a half and half for the first time last week. I had the same problem. The white was done well before the chocolate. I was extremely lucky because the white still tasted fine(I cut a small piece across the end to make sure it didn't taste dry).
I will bake two separate cakes and put them together in the future.
I like the foil idea-seems like it would work. The chocolate cake needs at least 10 min more. I ended up doing a 9" square cake of each flavor, put them together and trimmed off each end to make it 13" instead of 18". (to end up with a 9x13 cake) Worked fine and LESS stressful than both flavors in one pan!
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