Lightbrite, why should it be one word? Shmenoneh esrei is the feminine form of eighteen (“beracha” is feminine). Shemona asar is masculine. It is not possessive as the brachot do not belong to someone or something. It is a Hebrew grammatical form call “semichut” which joins two words. For example, “bet sefer”, “battei sefer” which join “bayit” (“battim”) and “sefer”.
BTW, for what is is worth in French it is “dix-huit” (lit. “ten-eight”).
Since we’ve been pronouncing it that way since before they added the 19th brachah, so the grammatical reasons for it are less important than the fact that is a well established custom
Akuperma, there was a certain shul where everyone did a dip at a certain place during the hakafot. They told the new rav who asked about it that it was a well-established custom. He checked the shul records and discovered that nce there was a low-hanging chandelier over that place.