Determining the fundamental mechanisms underlying tissue formation and wound healing allows for bio-inspired approaches that leverage the best practices of biology, as decided upon by millions of years of evolution. To gain insight into how tissue is built, we have studied the interactions between two of the principal proteins found in connective tissues: collagen and fibronectin. We have confirmed a binding affinity in solution, which supports a reciprocal relationship whereby the assembly pathway of one catalyzes the other. The molecular synergy enables complex extracellular matrix formation with a range of mechanical properties.