Tracking conversions for a 2-part form

I have a website that has a sidebar contact form that captures several pieces of contact info (let's call them A,B and C).

Once someone submits, it emails this info to me for follow-up and passes them through to our Contact Us page with a more in-depth form to collect additional info about their needs beyond just the basics. It also pre-populates this form with the data from A, B and C that they submitted on the sidebar form. So this form contains A,B,C,D,E and F.

Once they submit that they go to a thank you page w/ our AdWords conversion code.

Now...the trouble is, some people don't both filling out the more in-depth form once they've submitted on the sidebar, so while they've technically converted, AdWords won't pick it up because we don't have conversion code for the sidebar form in place.

Is there a way to do this that won't cause conversions to be double-counted?

So if they click an AdWords ad, fill out the sidebar and fill out the more detailed form, we would want it to just count as a single conversion. And if they just submit through the sidebar, it would still count as just one conversion.

Any suggestions on how to make this work? God I wish we could do conditional conversions w/ AdWords...