I think your best approach is to use 1 portal, with many http aliases.
Define 1 portal, with en.mysite.com and fr.mysite.com as http aliases. This way, since you have 1 portal, your users will be the same for both domains. You will not have to sync anything between portals.
You will then need something to change DNN language based on http alias. When accessing from fr.mysite.com, switch DNN language to french.
We have this kind of logic in our localization extensions, but we change language based on first part of url, not based on domain name (mysite.com/fr or mysite.com/en). Our extensions come with source code, this could be easily changed to accomodate what you need.
But I'm not too sure about using a subdomain to handle the new language for SEO. Each domain will be treated separately and have it's own Google page rank. I would rather consider using a single domain, with redirection of home to proper language, as our extensions enable, so that links to your domain for each language counts as a vote and increases the page rank for both languages.