  We consider the focusing energy-critical wave equation in space dimension $N
\geq 3$ for radial data. We study two-bubble solutions, that is solutions which
behave as a superposition of two decoupled radial ground states (called
bubbles) asymptotically for large positive times. We prove that in this case
these two bubbles must have the same sign. The main tool is a sharp coercivity
property of the energy functional near the family of ground states.
