How to combine two node/add/* forms into one form but create two different types of content in drupal 7 on submitting it?
My initial thought on a solution was also entity reference and entity inline form but a few caveats regarding the latter. The enitity inline form adds a submit for the additional content type, this seems against your proposed UX so you will need a hook_form_alter to change this behaviour (and indeed the label)  have a look at http://drupal.org/node/1880850 for a solution I worked on that will give you a lead in (but not a solution to) this.

The other issue is that I got a lot of annoying but non-fatal and non-damaging errors when I loaded entity inline form so stability may not be 100% there. This could be an issue.