Biological warfare? Just convince yourself to do it. The weaker both these empires are, the less resistance they can put up to the rightful rule of Rome. And when the inevitable happens (the occupation of Alexandreia, Antioch et cetera), you will not be forced to put the population to the sword - partly due to decimated population, partly due to the bigger garrison bonus, when you capture the cities.

Really, it is a win-win situation.