hi adrian,
One method to achieve this functionality would be :
1. Find out the maintenance screen and function group of the table maintenance generator say 0001 and Z03_PRO_FN. ( u can find out by going into tables - > utilities -> tmg - to find Function group. for screen number- from TMG ->utilites->modifications->maintanence screen -> note the screen number).
2. Go to SE80 and open Z03_PRO_FN screen number 0001.
3. For field 1 - create a module in Process on Value Request Event in PAI of the maintenance screen ( Similar to any module pool program)
call the FM F4IF_INT_TABLE_VALUE_REQUEST( In Process on Value request event of field 1 in PAI)
4. Use FM DYNP_VALUES_Update to read the value of field 1 on the screen ( Because this is before enter is pressed and field 1 on the module pool porogram would not be populated with value ) and update the field 2.
Regards,
Sivaganesh