You buy two knobs that are identical except for finish. Then you use the portions of each that you need.

At leas that is what I did in a similar situation. The time saved from trying to find what I wanted was worth the few bucks for the extra knob. Since I also wanted this split personality knob to match the other knobs in the hallway, I did this at the big box store where I could be sure that everything was the same in person (this was before you could order off the internet and hope you got the right items ordered/delivered)

Exactly as kempek said

Actually. Emtek, Schlage, Baldwin and other companies will do split finishes if you order the set that way.

Like Pal said, I have split finishes from Emtek (on my exterior doors, but Emtek does this for interior as well.)