Selection Screen - values selection based on 2 blocks

Hi experts,

                  Am an entery-level ABAPer.

                 I have 2 blocks in selection screen as shown in the attachment.

1st block - (Fields from PLAF table)

2nd block - (Fields from MARC table).

Common fields in these 2 tables are Material No(MATNR),MRP Controller(DISPO).

Now,

1) if I give inputs in 1st & 2nd block , 4 field values from 1st block needs to be selected and copied to an internal table where 1st table's MATNR and DISPO Equals MATNR and DISPO of the 2nd table(DISPO is a select option field in 2nd block)..

2)If I give inputs only in 2nd block, same logic as mentioned above.

3)If I give inputs only in 1st block , 4 fields' values should be retrieved from table PLAF and moved to my internal table.

SELECTION-SCREEN BEGIN OF BLOCK b1 WITH FRAME.

SELECT-OPTIONS : s_paart FOR plaf-paart,  "Order Type
                 s_plwrk FOR plaf-plwrk,  "Plant
                 s_matnr FOR plaf-matnr,  "Material
                 s_plnum FOR plaf-plnum.  "Planned Order Number
SELECTION-SCREEN END OF BLOCK b1.

SELECTION-SCREEN BEGIN OF BLOCK b2 WITH FRAME.

SELECT-OPTIONS : s_dispo FOR marc-dispo,  "MRP Controller
                 s_dismm  FOR marc-dismm, "MRP Type
                 s_mmsta FOR marc-mmsta.  "Plant-Specific Material Status
SELECTION-SCREEN END OF BLOCK b2.

I have just designed selection screen as shown above.

Please help me out on how do I achieve this. Post ur ideas and suggestions. Thanks in advance..