Text box linking

So I am trying to create a form that the text boxes link together, and so far ive run into a problem

I am using this script

var t1=this.getField ("Name of First Text Box");
if (event.fieldFull)
{getField("Name of Second Text Box").setFocus();}

And while it allows me to link them forward I can not backspace into the first field.

Is there anything I can do to make them backspace into the first field once in the second?

I am very new to java and i found this script so any help would be greatly apreciated.