Access denied when writing from derived tag

I have a long Int OPC tag and two separate derived tags - where one reads the first 2bytes of the long int and the other reads the last 2bytes of the long Int.
When I have security on the long int, the derived tags are unable to write to the long int - displaying Bad_AccessDenied in the output console.
I can modify the long int tag directly though so i know i have access.

It feels like the derived tags do not have the authority to write to the long int. Is there anyway to change this ?

Any help would be much appreciated.