Place cakes end to end on a 22-in. x 8-in. covered board. Level if needed. From the top of one end, form the front of the bus by cutting out a section 1 in. deep and 3 in. long (Save the removed piece for another use.)
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For frosting, in a large bowl, beat butter and shortening until fluffy. Add 6 cups confectioners' sugar, milk, vanilla and salt; beat until smooth. Add enough remaining confectioners' sugar until desired consistency is reached.
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