1215 has .09% carbon content, and very low manganese. It is close to the carbon content of wrought iron (.05%).
It isn't a blade steel.
The lowest carbon content blade steel normally used by knifemakers is .60% carbon (5160).
By "MIXING", if you mean layering it and getting a homogenous higher carbon final content, it really doesn't work that way. Carbon migration happens but it isn't like adding things in a kitchen recipe.
You would need to stack a 1.25" thick bar of 1095 to every 1/4" bar of 1215, forge weld it, and fold it a good 10 times to even get a 1080ish final result (which you would not actually get). ..... Math - [ .09+(5x.95)/6=.80 ]
Obviously, you would be far better to make knives from the plain 1095 and leave out the 1215.
The best choice of mixing would be to use it for san-mai. A cladding of 1/8" 1215 - 1/4" 15N20 - 1/8" 1215 would forge out to a nice san-mai billet with a bright and hard sharpenable edge and soft dark sides. This is similar to what many Scandanavian blades are made from.
An equal damascus mix of 15N20 and 1215 would make a high contrast billet that would be good for hardware such as guards and butt caps. It would not work as a blade material. It is decorative but would have poor edge durability. About 50-100 layers would look good.
You could combine those two uses of your 1215 and make an equal damascus billet of 1215/15N20 and use it as the cladding on a san-mai with 15N20 as the core. This looks quite stunning. I used to do it with 203E/1080 damascus cladding and 52100 core.