Can you tell the dimensions of this twin trunk?
The advises received are good.
You have other options in form and external design of the pot. I don't know about Boxwood, but you can plant this twin trunk over a rock slab, creating an scenario with small rocks, moss and sand.
Remember that the tree should define the pot and not the pot define the tree.
Pots are to complement the tree.
About the ratio of pot and tree, it depends in the tree style and appearance. A sturdy formal erect tree (Ficus, Black Pine) uses a larger pot than a delicate formal erect (Serissa, Azale
.