wire [17:0] bus_a, bus_b;
wire [17:0] mux0_bus;
wire [8:0] mux1_bus;
wire select_a, select_b;

assign mux0_bus = select_a ? bus_b : bus_a;
assign mux1_bus = select_b ? mux0_bus[17:9] : mux0_bus[8:0];

mux1_bus это выход