Sounds like it might be right, but without seeing the code it is hard to say.
The documentation to fill in a hdden field value is at http://developers.marketo.com/documentation/websites/forms-2-0/. The part that you need is
Set values on hidden fields on the form.
MktoForms2.loadForm("//app-sjst.marketo.com", "785-UHP-775", 1057, function (form){
//Set values for the hidden fields, "userIsAwesome" and "enrollDate".
//Note that these fields were configured in the form editor as hidden fields already.
form.vals({"userIsAwesome":"true", "enrollDate":"2014-01-01"});
});
Sounds as if you might be using just one form. So if your trigger is when that form gets submitted I could see how both campaigns would be triggered. I would suggest using two different form (it is the third parameter above). I think is largely a logic flow. So if I follow you, if i fill out one the other should be blank. So you need to do that in two seperate Smart Campaigns. It is having the success on a blank value if probably where the issue lays.
Brennan McAdams