typically the jambs are 1 1/4", so if you add 3" to 64" you come up with a rough opening of 67".
Problem is, is this interior or exterior, jamb sizes will change.
I just tossed some numbers at you, is no way accurate without knowing if interior or exterior, also possible the door manufacturer has their own specs. Need to check with them.
If you go big on rough framing, you can always fur it in later to correct sizing.
My numbers above only allow 1/4" each side for shims.
Depends on the opening on how much room you need. You may find the floor is not level along with walls and to fit door properly need a bit more room.
Simply not enough info from your post to really give a good answer, maybe my info will help.
Usually on a rough opening i go 2-1/2" on the weight and 3" on the height.So if you have a 32" door make it 34-1/2".Thats if iam framing it so i know its plumb level and square.
Now with double doors i would call the place you are getting the door from because there might be a divider between the doors.