Simple math exercise. But not for me)
80/20
To dilute 2mg/ml to 1mg/ml you need to half it, meaning you add the same volume of VG.
Here is a trick for same volumes:
The liquid you have is 60/40 and the liquid you add is 100/0 so a simple trick is to join them and then "fix" the ratio to sum as 100. Int this case division by 2. Meaning: 60/40 -> 160/40 -> 80/20.
Another approach is to pick a convenient volume, such as 100ml. So you know 60ml are VG and 40ml are PG. Adding the same volume as VG will yield 160ml of VG, so 160ml out of the new total of 200ml are 80% (160 / 200 = 0.8)
What's the target volume?
It would have to be twice the starting volume, so the starting volume doesn't matter here.
Right, target or finished volume.
I guess I was trying to help OP with a method that involved 0 math, but wasn't sure of the mixing goal.
The way I would do it would be to split the original bottle in half into 2 bottles of the same size as the original then top the new ones off with VG leaving 1mg/ml and a ratio of 80/20 VG/PG.