Trim the top and sides of one cake to straighten them, if needed. Spread a bit of frosting onto the center of a serving plate and set the trimmed cake upside down on top of it (to keep the cake from sliding).
Trim the sides and top of the second cake to straighten them. Then slice off a 1 1/2-inch-thick piece from one end and use the remaining portion for the top of the bus, frosting the bottom and setting it atop the first cake.
Spread a thin layer of white frosting over the whole cake to seal in any crumbs and let it set for about 15 minutes. Spread more white frosting on the sides of the top layer (the windows of the bus). Reserve about a teaspoon of white frosting, then tint the rest yellow and use it to frost the rest of the cake.
For tires, use dabs of frosting to attach white chocolate chip hubcaps (tips down) to the centers of the mini doughnuts and then to stick the tires onto the bus.
Transform cookie wafers into people using white frosting and black gel to create eyes. Trim some red gumdrops for smiles, then press the cookie faces onto the cake.
Finally, add details such as black gel lettering, Red Hots for caution lights, and white chocolate chip headlights.