Multi-selection combined with an editbox in the same custom field?

Hello

With my existing registration form, users can (1) select a marathon name into a listbox then (2) select how many times he ran it then (3) click on a 'Add' button to add the couple 'marathon name / occurence' in a 'marathon ran' list
See the screenshot attached.
The beauty is that we have only one listbox (in reality 2 for France & Abroad marathons).

When registred,
- we can display this list in his profile like
. Marathon de Paris (x3)
. Marathon de Chicago (x1)
. Marathon de Londres (x6)
- we can display the sum of his marathons under his picture in discussion forum, articles he wrote, etc.
. Henri Lacroix (10) : which is very appreciated by our community runners

With your Custom Field, the workaround I see is to create many many couple listbox (400 marathon names) and occurence editbox.
- the user will take the 1st listbox, select 'Marathon de Paris' and edit '3'
- then he will take the 2nd list box, select 'Marathon de Chicago' and edit '1'
... etc.
but some of our members ran 150+ marathons, average is 30/40 ...

With my existing form, there is only ONE (in reality 2) listbox.


Any ideas ???

ps: if you want to see how it works, feel free to register to http://www.courirlemonde.org