You don't need to do anything on the server to accomplish this: a Marketo form can create more than one lead. This is how we create referrer programs, and I'm sure I've posted a(nother) demo here in the past few weeks.
One other thought I had was using custom objects. I haven't used them from Salesforce before. Would there be a way to create a custom guest object in Salesforce which syncs to Marketo?
COs won't help at all with this goal.
An alternative is to use smart campaigns and to fire off webhooks to your own marketo forms endpoint while copying the data from your custom fields to primary fields