Restrict custom field modification specific workitem type

Hi All,

There is a feature in RTC which can,infact, restrict the modification of a particular field in a workitem. Problem is that, i have a custom attribute that i use in two different workitem types. I want to restrict the modification of the custom field to a particular role and only one work item type. But, now when i do that, it affects the other one too.
Can someone tell me how to restrict the custom field modification to one workitem type?

Many Thanks.