The database and models are all hooked up for add-on dependencies already but we don't reveal that on the front end at all. Please find on the edit add-on listing page to give authors the opportunity to declare add-on dependencies. There should be some reusable chunks of UI in the admin tools (auto-complete for add-ons search and cvan's feature editor page come to mind). There is the potential for cross app/add-on dependencies so let's not restrict add-on type I suppose. I thought chowse had mockups for how to reveal this on the detail pages but I don't see them now. Regardless, we can solve that in another bug once this is hooked up.
the mocks are linked from the marketplace spec
found a mockup! http://moz.howsehold.org/amo/devtools/v3/manage_addon/03_edit_listing_expanded.png I see fligtar already commented 20 minutes ago, but I'm doing it anyway.
waffle flag is edit-dependencies; enabled on -dev