there'll be 2 check boxes in one row, with same function (only one can be clicked and when so the text field opens)
You can place two radio buttons in a single RadioButton group on the form and change the style to Solid Square. That way they look like checkboxs but only one can be selected.
To open a Text Filed, you can place a TextField on the form and then set the presence property to "hidden" or "visible" based on the Radiobutton click event.
Language : JavaScipt
RadioButton Click Event
if(this.rawValue == 1){
TextField1.presence= "visible";
}
else{
TextField1.presence= "hidden";
}
Hope this gives an idea..
Thanks
Srini