Etsy's CS is not set up like this. I don't know of any online site that is set up like this.
You can sometimes tweak your weights to make it work out better.
I know both Kathy (woodcraft) and I do this for multiples to work out.
In my other shop I sell winter goat coats and some sizes I sell, weigh with a box around 1 lb. 10 oz.
If I put 1 lb 10 oz for the item weight and someone buys 2 coats, CS figures at a 3 lb. 4 oz rate.
But I can get 2 in a slightly bigger box for under 3 lbs.
So what I do is I put 1 lb. 6 or 7 oz for the single item weight.
So when 2 are purchased CS figures on a 2 lb. 12 to 14 oz box.
It doesn't matter on a single item weight if I have 1 lb. 7 oz or 1 lb. 10 oz, the shipping rate is the same thing
I've been doing this method for years on Ebay and it works out for what I'm selling in both of my etsy shops.
Ebay figures "total" weight also, no where to add just the box weight separately.