How to call two methods on button's onclick method in HTML or JavaScript?

Try this:

<input type="button" onclick="function1();function2();" value="Call2Functions" />

Or, call second function at the end of first function:

   function func1(){
     //--- some logic
     func2();
   }

   function func2(){
    //--- some logic
   }

...and call func1() onclick of button:

<input type="button" onclick="func1();" value="Call2Functions" />