Make cake according to package instructions, adding the mashed bananas and strawberries to the mix. Pour half of the batter in pan, then add sliced strawberries and bananas in a single layer. Pour other half of batter in pan and bake according package.
While the cake is cooling mix JELL-O according to packages.
Poke holes in the cooled cake. Add a layer of sliced bananas and strawberries.
Pour JELL-O over cake and chill until JELL-O is firm.