When you have two items, the first item is simply the first item. However, the second item implies there might be more after it, and the last item doesn't tell you out of how many.
Is there a single word that means both last and second, or last of two?
A is the first item.
B is the ___ item.
Other is no good, as it is usually paired with one and it also doesn't specify an order. One could be A or B, in which case the other one is B or A.
I'm working with something like an ordered pair, but it's not obvious that it's a pair of items. It's to do with words regardless of the language, and in Semitic languages, the left word is the second word and the right word is the first word, whereas in English the order is the reverse of that.