It seems my test has confirmed you cannot have more than one carrier-activated phone regardless of model, color or memory capacity. If the two devices are on the same carrier network you can only have one in checkout for your order. If the phones you have are of different carrier networks you are allowed to add one (in the same checkout transaction) and only ONE of its carrier type again regardless of model (6, 7, 8, X), color or memory capacity. [Example: one Verizon activated iPhone, one {different carrier} activated iPhone, but never more than one of the same carrier type.]
This simply applies to my tests on the iOS apple store app, may or may not apply to website as I haven’t tried in that method as of yet.
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Yes this screen shot posted confirms that you can buy 2 sim free models simultaneously as shown in your picture. Carrier-activated models (which is the only selectable option for iPhone X) won’t let you get past one if you test it for iPhones already available (tested with same 8+)