Sure, in some sense :) This model exists under a number of names: Pesero, Jittney, Monit Sherut (to name a few).
That said, it's not just an app that we've built. It's a system that can make dynamic decisions about which routes merit being paired together, and communicates with our drivers about any changes that need to be made to the current route. Of course, we take on-road safety into account, and so changes are only made while the vehicle is stationary.