A few more months and we could have celebrated the one-year anniversary of this thread
What maerean_m, who is a member of the dev team to those who didn't know, said is correct in one sense: You can only man one deck gun yourself, no matter how many are installed.
You can however install as up to four guns of any type by editing the correct files. Weight and drag aren't an issue because the dg has no effect on them. If it had this sub would be in big trouble:
That's four of Yamato's turrets on a Balao