We created a new Marketo instance. In a staging environment we have added Munchkin tracking to both the new and old instance via: <script src="//new.mydomain.com/js/forms2/js/forms2.min.js"></script>
That's not actually Munchkin tracking, it's the Forms 2.0 library (which uses Munchkin if it's present, but doesn't depend on Munchkin).
Most likely you're having the known problem where you try to embed forms from 2 different instances; this requires more work than just adding the library twice (it actually will only create a single global object that way).
Please provide a URL and I can tell you exactly what's going wrong.
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