You're right - each side mirror is supposed to have 2 studs, and the mirror is held to the door using two nuts (use a 10mm box end wrench). I haven't ever had the trouble you describe (a stud missing), but would assume that when you remove the mirror, you will find there is a hole in the base to receive the stud. If so, you could just epoxy a stud in its proper place.
It's possible the stud fell down into the door - might want to look for it there. That way, you avoid an annoying rattle and also avoid having to fab a stud
!