Send signal from one output connected to multiple modules
I presume you have your input and output pins in the main module swapped.  I also presume that the main module should have both c and d in the definition.  I presume you are asking how to assign the outputs of the modules to the d output from the main module.  Here is the simplest way to implement that:

wire c;
wire d1;
wire d2;
reg d;

reg select;

one mod1 (.a(c), .b(d1));
two mod2 (.a(c), .b(d2));

always @(*) begin
    if (select) begin
        d &lt;= d1;
    end else begin
        d &lt;= d2;
    end
end


Change the always statement to a clocked always statement if you want to register the output.