They have different names, within the same line. Even though they look identical to each other, they are different toys.
Same with Swoop from the Armada era Dinobots...he looks the same as Terranotron, but because they have different names on the packaging, they are different figures. Initial shipments had the name Swoop on the file card/sticker and command a much higher value than the later shipments that replaced the Swoop card with the Terranotron card (I believe they lost the rights to the name Swoop and actually used it "illegally" on the initial shipments). Yes, physically, they're the same, and post-package opening, you're right - unless you had the card(s)/packaging, you could not tell one from the other.
Another one off the top of my head would be RID Spy Changer Ironhide (from the second series of exclusive translucent Spy Changers) and Universe Spy Changers Hoist (2nd series). Both are physically the same figure, but there is a distinct name change, so they are different figures. They're intended to be different characters, they just happen to look identical.
Now, in a somewhat-related vein, when considering Canadian Generation 1 figures...even though there is a French name on the packaging, as in
Snarler/Grondeur, we link the Snarler figure from the U.S. package to the Canadian package. Why? Because the figure is still known as Snarler in Canada with Grondeur being a secondary name/translation for the French-speaking contingent of Canada - there isn't a true name change.
This is different than the Billy/Jack situation as this spans two countries, not being limited to only one line (and one country), like in the case of Billy/Jack. I suppose a note explaining how Billy and Jack are physically the "same" figure in the details section of each might help.
MIKE
engledogg