Depending on which COBOL you are using, if you only use the field name it may default to the first one defined. However, virtually all flavors of COBOL support a qualification of COBOL field names and you probably want to use that syntax..
05 FIELDA PIC X.
05 FIELDB PIC X.
05 FIELDA PIC 9.
05 FIELDB PIC 99.
MOVE SPACES TO FIELDA OF IN-REC.
ADD FIELDA OF WORK TO FIELDB OF WORK.