Forms: Post to another Mautic form?

I have two Mautic instances (installed on the same VPS).

I have created an identical form on both instances (the same fields that are named the same thing). The from at a.mautic.com has the form action to post the results to b.mautic.com.

When I fill out the form at a.mautic.com the form submits successfully, however nothing happens at b.mautic.com. If I manually fill out the form at b.mautic.com the form works fine.

What am I likely doing wrong?

I don’t have an authorization header, though I can’t imagine why I would need one…