Okay, I am thinking if a player can be mounted to turret, can I mount a turret to turret since turret has same datablock as the player.
Then I tried it. then I end up something like the picture shows below.

This looks okay at the first place, but when I ride the tank, I can't get in the turret by using changing seat server cmd.
I implemented the onTrigger function to get myself in the turret, the changing seat cmd works in the tank turret and I can get in the MG turret.
However, I got a console spawn--Warning: Multi-seat turrets are not implemented yet. so how can fix the problem? or we are not allowed to have turret on turret?
PS: the MG that I used for testing is "borrowed" from T90 made by Gravity Cat. If this problem can be fixed, I will make my own MG for sure.