Unfortunately the eBay system doesn't have a function to differentiate purchases as such in the same listing.

You could have at the beginning of the description NOTE - International buyers minimum purchase is 2 items.

Or you could retain your listings as minimum 1 but change the postage costs in a policy to reflect tracked postage for International purchases.

Thirdly perhaps have a set of trial listings with the more popular designs sold in pairs for International buyers and list them on the USA site.

I have found some items sell better on the USA site.