So basically I have the backpack plug-in

I have already given players, 1 backpack with the size of 6 rows ( the max backpack )

Now I want to give 2 backpacks with Max size of 6

So I want them to use /backpack 1, /backpack 2

Like that how could I do this?

I am afraid, this is currently not possible.
There are plans for multiple pages, but it is not yet done.