I am making a half sheet for 35 people.  They want half vanilla and half chocolate.  Thought about making a strong aluminum foil dam down the middle and place the different batters on each side.  Any other ideas?

Couldn't you have layers instead- one layer chocolate, one layer vanilla? That way everyone gets one of each flavour.

AThat is what I do when I am making a 9x13. I am only making 1 level and not sure if you are doing two levels.

They want the flavors side-by-side.

Bake a single layer of each flavor, cut in half and stack.