My friend wants two cakes in one sheet cake, she wants half carrot cake and half French vanilla cake. How can I make this?

Put two half sheet cakes side by side.

I usually prop one end of the pan on a cooling rack, and pour the 1st flavor in the lower half. Then as I pour the 2nd flavor in the other half, I lower it. Make sense?

I, too, would bake 2 separate halves and place them together. The carrot cake is likely to need more time in the oven than the vanilla - you may wind up with one underdone and one overdone if the two batters are in the same pan.

typically i would just bake both batters in the same pan, but i agree with the pp, carrot cake usually bakes differently than other flavors so i would also bake in two seperate pans and place together on the cake board. Then ice as one cake.