minhag - Splitting up Shacharis between two Chiyuvim

I have Davened in a few Shuls where they have split up Shacharis between two Chiyuvim. (One Davens until Ashrei after Tachanun / One davens from Ashrei until the end.) What is the source for doing so?

Answer

Beur Halacha 132 discusses drawing lots to determine who leads the service and then adds (in my own loose translation):

… and all this is when they can't both lead the service; if they can, and both are acceptable to the congregation, then let them draw lots [only] to determine who leads until "Ashre–Uva l'tziyon" and who leads from there on.