How to write a CCK widget with more than one input element?
I built something like that for drupal 7. Considering CCK moved into the drupal core in d7, I'm not really sure how much different the code would be for a cck module in d6, but maybe this module will help you

I wrote this module as I had to somehow capture phone numbers, but have them split into country code / area code and number and save them into seperate columns in the database. All other field modules I tried did not allow for this!

That way all numbers can be entered uniformly and thus presented in a uniform way - especially helpful if you use it to store and manage customer information and possibly need to export it to different applications.
In my case, some users would enter phone numbers with a dash, other with a slash and others again with nothing seperating values making managing them difficult and displaying them messy.



Rip the code apart and modify it to suit your needs (like only 2 fields):

It's in my sandbox: http://drupal.org/sandbox/tecjam/1320886

in Git: http://drupalcode.org/sandbox/tecjam/1320886.git/tree/e7c97511506ee6e92a44656aa02d31f4e8f69727

I like to comment my code to help myself (sometimes there are more comments than code lol). So figuring out what does what and where should not be a problem at all.

You can always ask if you have any special questions.