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”).