We study the propagation in an empty but favorable environment of two competing species. They both grow and compete following a logistic equation, such that coexistence is unstable. We will show that the fastest species does in fact propagate at its Fisher-KPP speed, and depending on the parameters, the slowest one replaces it or not at at lower speed. This system models the propagation, in a Petri dish, of two populations of cancerous cells.