I read this book back in the early 90s. It was about a pair of wizards - the older mentor and younger student. In their world wizards are generally looked down upon and even have fewer rights than other citizens. They live in a kingdom that is more enlightened about such things and work for the local king. They perform mostly low magic - one of the spells I recall them using was "Look over there!" or some thing to that affect that caused people looking for them to look in another direction. I remember that they surmised that they could make stronger magic if they had a way to purify their ingredients better.
The younger wizard had illegitimate children with one of the kings daughters. I recall a scene where they converse with a "blood mages" that uses aggressive magic.
At the end of the book they have been summoned to earth, where there is no magic.
Does anyone know what story this is?