Our customer asked:
Yes it should be perfectly safe to use these two products together because they work in completely different ways and contain completely different ingredients, with no known interactions to my knowledge.
I would say that theoretically you should not need to use them together, because Advocate should deal with fleas on its own. However if there is a heavy burden to get rid of, the Capstar should provide a dramatic immediate effect. The general environmental population in the background might act as a reservoir for a while, so an environmental flea spray such as Indorex could be considered.