The ${\text{Nd}}_{1-x}{\text{Ce}}_x{\text{CoIn}}_5$ alloys evolve from local moment magnetism $\left(x=0\right)$ to heavy fermion superconductivity $\left(x=1\right)$. Magnetic order is observed over a broad range of $x$. For a substantial range of $x$ $\left(0.83\le x\le 0.95\right)$ in the temperature-composition phase diagram we find that superconductivity may coexist with spin-density wave magnetic order at the Fermi surface. We show that a delicate balance betwen superconducting and magnetic instabilities can be reversibly tuned by both the Ce/Nd ratio and magnetic field, offering a unique model electronic system.