when i do half and half i roll a few dish towels and place under the pan so it is at an angle then i fill with one flavor then i start filling with the other flavor as i remove the dish towel the thick batter of the first flavor takes a few minutes to level out and that allows you to pour in the second flavor hope that makes sense
I wrote this in another thread:
A commercial Bun pan is 18" x 26" (outside measurement), and because they are tapered for nesting or making them stackable, the inside measurement is 16.5" x 24.5".
A commercial full sheet is 16" x 24". They are baked in 16" x 24" bakeable cardboard trays that fit into the Bun pans (flat surface portion) which are used during baking for support and handling purposes.
A true commercial full sheet (16" x 24") serves 96 (unit wt. 106-124 oz.).
If this size pan doesn't fit in your oven, and you are baking two 12" x 18" (54 serving) half sheets, they would serve 108 total.
I only have a 11x15 and a 12x18 pan. so I should do one 12x18 chocolate and then another in vanilla cut down one side on each of the longer size so they are straight side by side and lay them next to each other? i dont think a full sheet cake pan would fit in my oven. how do i go about icing this cake? i normally let my cakes sit in the freezer to get the icing to firm so i can do the crumb coat and then a couple more layers to make it smooth. if i cant get the cake into my freezer which im sure I cant how do i go about getting the icing to crust properly?
How much butter is in your frosting? Because you may not need to freeze it to get it hard. I refrigerate mine for a bit and do the smoothing thing on it...but my BC uses just butter and gets pretty firm just in the fridge. I don't use shortening but I imagine that may not get as firm in the refrigerator...but then you really don't need it to be all that firm if all you're doing is frosting over a crumb coat.
If you just want it to crust leave it out and it will crust on the outside but still be soft on the inside. That's the best time to use either the Viva towel method for smoothing or the roller method for smoothing.
i did a half and half cake like you're talking about. It was two 9" squares put together. If you're putting the pieces together you will have to make sure to level each cake so they will look like one. Are you coating in just one frosting or doing half and half on that as well. Here's my cake and how I approached that:
(please forgive the lousy photo...and the fact that this was one of my earlier endeavors! ) http://cakecentral.com/gallery/1836000/sheet-cake
This was a one-layer cake. Had I to do this again I would have stacked them two layers with a filling to make it taller and cleaner looking.
Here's one I did recently only with a round cake. You can see how I approached the half and half issue with this one:
http://cakecentral.com/gallery/2271073/half-chochalf-vanilla-birthday-cake.
HTH! 
Does a 12x18 fit in your freezer? If not, chill them in the refrigerator. When you're ready to crumb coat, place them together side by side on your final cake board and ice.
If it doesn't fit in the refrigerator to chill before applying fondant, you could:
stack them as a layer cake instead of side by side,
use a neighbor's refrigerator or freezer,
apply fondant immediately after icing without re-chilling, or
buy an extra refrigerator or freezer.
(these are the only options that I can think of... perhaps someone else will have another suggestion.)
I take a thin piece of cardboard wrapped in tinfoil, cut to size to divide my cake pan in two. I pour one flavor on one side, then the other flavor on the other side, and then remove the cardboard. You get a little crossover of flavors in the middle, but otherwise it works well for me.
Hope that helps!
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