How to build an 8 output demultiplexer, using only the two output demultiplexers?
You can build the binary tree of muxes. First mux has outputs connected to 2 muxes, providing 4 outputs, next stage will have 4 muxes. Total you will have 7 muxes in 3 stages. Each of 3 address inputs is connected to inputs of corresponding stage. (Just revert the picture below upside down, so it were demuxes not muxes)