Malte,
There's no way to do what you need with the actual ad_form and formbuilder (AFAIK).
For that particular case and a quick solution, you can have a look at template::widget::date and do something similar.
About to add a doubleinput widget, I don't think it's the right way to go. Why double only? A triple one might be useful also (and would allow to rewrite the date one without using table for instance). I mean there's no reason for a fixed number of elements to display inline -however a limit would be necessary-.