I have Leads/Contacts that have 2 email addresses. We have a field in the Contact for an alternate email. I was under the impression that Marketo can only look at the primary email when looking for duplicates. Is there any way to look at both emails when they come through the web form so it does not create the duplicate Lead?
So despite my usual confidence, I'd say "no" here as well.
You could write an interstitial proxy layer on your server (thus not using Mkto forms anymore) that uses API calls to accomplish this. I don't advise it.