Hi edwin,
you are speaking about 2 content type.
When you are in a form of 1 content type, you can't access directly to fields of an other content type.
The only way is to request the database, depending where you do the process.
If you are in the form of the content type where you grab the value, you will make a request UPDATE on the table of the other content type.
If you are in the form of the content type where you set the value of a field depnding on an other content type, the request will be a SELECT on the other table. In this case, after the request, you can store data like the tutorial below show you.
Regards.
Lionel