Given the meaning of "uremia" (a form of blood poisoning), "polyuremia" should not be a synonymy of "polyuria" (excessive urination). Uremia is much more likely to result from too little urination than too much. It's not clear that anyone has used it in the sense of "multiple forms of uremia in one individual", which is what it would be expected to mean.
But I wouldn't expect the word to be used in a chemical encyclopedia, unless it already existed in the medical literature. The couple of occurrences I've found there look like errors for "polyuria". For example in the abstract of this article in PubMed, "polyuria" and "polyuremia" seem to be used interchangeably.