Because you allow for
all fields minus those within "b".
when submitting "a", the following would work, using regular web-forms without fancy JavaScript tricks:
Step 1. Put each form on its own web page.
Step 2. Insert an iframe wherever you want this sub-form to appear.
Step 3. Profit.
I tried to use a code-playground website to show a demo, but many of them prohibit embedding their websites in iframes, even within their own domain.