// declare the following vars wherever you do that in your mod:
gamevar x 0 0
gamevar angvar 0 0
gamevar savx 0 0
gamevar savy 0 0
gamevar newx 0 0
gamevar newy 0 0

// in the shooting part of the code, do this:
shoot RPG
geta[].x x
geta[].ang angvar
sub angvar 512
add x 384
rotatepoint sprite[].x sprite[].y x sprite[].y angvar newx newy
geta[].x savx
geta[].y savy
setsprite THISACTOR newx newy sprite[].z
shoot RPG
setsprite THISACTOR savx savy sprite[].z

If the position on the second RPG shot is wrong, change sub angvar to add angvar. You can also play with the distance 384 to make it smaller or larger (that one is the gap between the shots) 