making 16x1 multiplexer by using two 8x1 multiplexer and one 2x1
You didn't show where the select-lines of upper mux is connected to. If those select lines are connected to the select lines of lower mux, then it is correct. 

From the figure, it is not quite clear that which line is connected to select-line of 2:1 mux. If you are connecting MSB of the select-lines to 2:1 mux, then the address of the channel will be from top to bottom as Mister Mystère commented. If you are connecting the LSB to 2:1 mux, then the upper mux will be carrying the even addressed channels and lower will carry the odd numbered channels.