Hi try This,
CASE SY-UCOMM.
WHEN 'TAB1'.<function code of the Tab>
TABS-ACTIVETAB = 'TAB1'.<name of the Tab>
WHEN OTHERS.
TABS-ACTIVETAB = 'TAB2'.<Name of the Tab>.
ENDCASE.
and call the both subscreens in PBO
PROCESS BEFORE OUTPUT
MODULE STATUS_0100.
CALL SUBSCREEN SUBS1 INCLUDING SY-REPID '0101'.
CALL SUBSCREEN SUBS2 INCLUDING SY-REPID '0102'.