you will have to butt them, not miter
As stated the bevel edge will be longer than a square edge. So I would butt them, that is lay the bevel on top of the surface of the 90 degree board. The biscuit would be at 90 degrees on the square edge and at 90 degrees to the bevel on the other. That will be the challenge. You will need a jig to hold the beveled board 11 degrees off the table while you run the biscuits in.... nothing fancy just a support with a brace. Hot glue is your friend in this case. Just a dab will secure the board and then lift it to the 11 degree, run a scrap under it and hot glue that down.
I run my biscuits using the base on the table surface as the platform, not the fence working from above. I don't like the fence gettin' in the way... that's just me.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)