The example below  read 50 records at a time from VBAK into an internal table, and selects the corresponding entries from vbap into an internal table. Then the two internal tables can be processed, and the next 50 records from VBAk can be read. Remember to reinitialize tha tables before the next read. 

REPORT  z_test.

TYPES:
  BEGIN OF t_vbak,
    vbeln LIKE vbak-vbeln,
    erdat LIKE vbak-erdat,
  END OF t_vbak,

  BEGIN OF t_vbap,
    posnr  LIKE vbap-posnr,
    matnr  LIKE vbap-matnr,
    meins  LIKE vbap-meins,
  END OF t_vbap,

  BEGIN OF t_report,
    vbeln LIKE vbak-vbeln,
    erdat LIKE vbak-erdat,
    posnr  LIKE vbap-posnr,
    matnr  LIKE vbap-matnr,
    meins  LIKE vbap-meins,
  END OF t_report.

DATA:
  li_vbak   TYPE t_vbak OCCURS 0,
  l_vbak    TYPE  t_vbak,
  li_vbap   TYPE t_vbap OCCURS 0,
  l_vbap    TYPE t_vbap,
  li_report TYPE t_report OCCURS 0,
  l_report  TYPE t_report.

START-OF-SELECTION.
  SELECT vbeln erdat
    FROM vbak
    INTO TABLE li_vbak PACKAGE SIZE 50.
    SELECT posnr matnr meins
      FROM vbap
      INTO TABLE li_vbap
      FOR ALL ENTRIES IN li_vbak
      WHERE vbeln = li_vbak-vbeln.
    IF sy-subrc = 0.
*   Now you have the two internal tables li_vbak and li_vbap filled
*   with data.
*   Do something with the data - remember to reinitialize internal
*   tables
    ENDIF.
  ENDSELECT.

*-- End of Program

Fast Links:
Using Select-Options and Ranges
Difference Between Select-Options & Ranges

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