Add a custom field on click of Add Another item button

Hi,
I am trying to add a custom field under a cck field with unlimited values.
I use hook_form_alter to add the field and its working fine. But when i click on Add Another Item Button only cck field is added via ajax.
So its like this
showing cck field + custom field by default

but on click of Add Another item Button
it show only cck field

Please tell me how can i show custom field with cck fields created by Add Another item Button.