How to make a CCK field appear as single-value on one Node Type, and multi-value on another Node Type?
You need to implement the hook_nodeapi and use the operation "presave"

Presave will have all the data submitted from the form and you can then alter it here just before it goes into the database.

If you have the devel module do a dpm($node); to see what data you are getting.

the important thing is that the field for multi values will be expecting an array so it may be as simple as grabbing the data from the form and adjusting it to pass it as an array.

EDIT

Aah ok validation....

I would probably turn off validation for that item by using the: '#element_validate' property, this way you can target validation for that specific element see the fapi docs on drupal. 
http://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/6#element_validate