I think the word "semantics" is singular (just like e.g. "mathematics"), at least it is used that way (e.g. "… the semantics of … "). I know its meaning at least in technical terms and I understand that often it makes sense to "treat it as singular" such as stated here. However, I definitively need the plural form and it feels like "semantices" sounds terrible.
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I insist that "semantics" is countable in the context at hand. I use "a semantics" as a mathematical term that refers to basically a function that maps strings of symbols to mathematical objects. Those functions are a countable quantity (in the linguistic, not the mathematical meaning of the term).
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I do understand that "semantics" is already in plural but this doesn't solve the problem at hand because, if what Jon Hanna says is correct, then it "doesnt work for something countable".
Best Answer
If you need a countable plural corresponding to a semantics, just use "two" (three, four, etc) semantics. Nothing else is possible in terms of a single word (you are correct about "semantices" being awful; the same goes for "semanticses").
Using "semantics" as a countable noun seems jargony to me, but it is precedented:
Google Books "two semantics"
You would do the same thing if you are talking about different kinds of "mathematics" – e.g. "I will discuss two mathematics: the mathematics of X and the mathematics of Y".