You'll need to grab the parameters from the URL into cookies which will travel with the lead past the interim LP and to the form LP -- you can then grab the cookie values into hidden field on your forms. There some fairly simple javascript to do this although you'd need help from a developer.
Taking it a step further, if you use SFDC campaigns, you may want to consider child campaigns in SFDC for each source (email, banner ad) underneath each of the two programs so you can track each promotion channel separately.
Check out this post from Josh Hill which is relevant:
http://www.marketingrockstarguides.com/marketing-attribution-setup-checklist-1846/