only one database can be referenced in an RSE or an expression

  Explanation:  You cannot use database variables from  more  than
  one  database  in an RSE or in an expression.  You can, however,
  refer to variables from one database in an expression and  refer
  to  variables  in  another  database  in  a  STORE  or  a MODIFY
  statement.   Nested  FOR  statements  may  refer  to   different
  databases in the RSEs of each FOR statement.

  User Action:  Use separate statements or nested  FOR  loops  for
  multiple database access.