[felgall]

Best practice would be to move the JavaScript out of the HTML input tag and into a separate JavaScript file. Simply give the button an id and then you can attach as much onclick JavaScript as you need in the JavaScript file rather than in the HTML.

<input value="Calculate" id="calc" type="button">

document.getElementBiId('calc').onclick = function() {
   a(document.form.v1.value); // or whatever JavaScript you want
   b(document.form.v2.value);
};

or better still use an event listener instead of the event handler so that other independent actions can be attached to the same event.