Say the select lines are S3 S2 S1 S0 , use txo 8x1 mux and use S2 S1 S0 as select lines for both of them , now connect the output of two mux to another 2x1 mux with upper 8x1 mux output as 1st input and next one as second input , now use remaining select line (S3) as select line for 2x1 mux , take the output from it..
Say the select lines are S3 S2 S1 S0 , use txo 8x1 mux and use S2 S1 S0 as select lines for both of them , now connect the output of two mux to another 2x1 mux with upper 8x1 mux output as 1st input and next one as second input , now use remaining select line (S3) as select line for 2x1 mux , take the output from it..