Is it valid html to have the following:
No, it is not valid but you can trick your inner form position on the HTML
with a help of CSS
and jQuery
usage. First create some container div inside your parent form and then in any other place on the page the div with your child (inner) form:
....
Give your container div some stable heaght
Thank trick the "other_form" position relatively to container div.
P.S.: You'll have to play with numbers to make it look nice.