I'm an illustrator who writes novels. How do I query?

Yeah, I think if the two mediums aren't combined (a graphic novel or similar), you'd have to pursue each area separately.

I'd want agents who were expert in their fields. It's not likely that a literary agent would be expert in placing both books and illustrations, as the two mediums are so different: so yep. Two different agents.

I wasn't aware that agents had much to do with illustration work.

Most of the agents who handle illustrators (and it's not so different or unusual) handle children's book authors and illustrators.

Personally I would recommend querying JUST the book and query agents who take authors and illustrators. Then talk to your agent about illustrating the book.

There are also folks called artist reps who rep illustrators only.

Decide whether you are more passionate about the art or words and seek representation for that one. I think most reps expect the artist to work in picture books or the educational markets. You'll have to do some research on that.

You can also query the book at agencies where someone reps illustrators. Folks in the same house may share you.