Butter and flour 2 (9-inch) round cake pans with 2-inch high sides. Combine cake mixes, water, oil, and eggs in a very large bowl. Beat for 3 minutes, or until well blended. Divide batter between prepared pans. Bake for 40 minutes, or until toothpick inserted into center of cakes comes out clean. Cool cakes in pans on cooling rack for 20 minutes. Invert cakes onto cooling rack and cool completely.
Cut 1 cake horizontally in half. Place top cake layer, cut side up, on serving platter. Spread 1/2 cup of jam over cake layer. Top with bottom cake layer, cut side down. Cut second cake vertically in half, forming 2 half-circle pieces. Spread remaining jam over top of 1 half-circle cake, Top with second half-circle cake. Place stacked half-circle cakes on top cake on platter. Spread entire cake with frosting to coat completely. Arrange purchased decorations on cake to resemble circus stage.