repost .asp form with data

2 scenarios on same form that I can't figure out.
1) I have 2 text boxes on initial form, txtUPC1 and txtWT1. User keys information into the boxes, then can click button to add 2 more text boxes, txtUPC2 and txtWT2 returning to same form with data in txtUPC1 and txtWT1 preserved, and now having txtYPC2 and txtWT2 available for data entry.
2) User keys customer# in txtCN1 then clicks button. function retrieves data and populates txtCN2... back to same form always preserving all data previously keyed in form.
Don't need someone to code this for me, just point me in the right direction please. Thanks!
Paul