CALL FUNCTION 'REUSE_ALV_GRID_DISPLAY'
EXPORTING
...
I_CALLBACK_PROGRAM = z_variant-report
(or your program name, not sy-repid because this will
change during the use of ALV Grid)
I_CALLBACK_USER_COMMAND = 'USER_COMMAND2'
(or some other name)
...

then you define your form:

form user_command2 using r_ucomm like sy-ucomm
rs_selfield type slis_selfield.
case r_ucomm.
when '&IC1'. (doubleclick)
if rs_selfield-tabindex > 0 and
rs_selfield-sumindex le 0.
read table itab into wa index
rs_selfield-tabindex.
perform second_grid.
endif.
endcase.

then your form for the second grid:
form second_grid.
(here you can make your selections for the second grid
based on the row you selected into workarea wa from the
first grid, and then call REUSE_ALV_GRID_DISPLAY again,
with another field catalog, internal table etc...)
endform.