Post Specific Form Vars Without Submiting?

I have now a form inside a form which of course doesn't work. I need to have a way of getting specific field data without posting till the whole form is done.

For example,

the form contains several items but a specific section for part quantities allows the user to enter the quantity and then click on the "Add Part Nums" button to get a form in a popup that has entries for part numbers based on what the quantity was in the first form.

I have this part working perfectly but it's a form inside a form so it only works when I comment out the outer form tags.

Is there a way to submit the quantities fields only and get the pop up and then later submit the entire form? A way to do this without having a form tag for the inner part?