  The origin of the spin gap in the underdoped cuprate superconductors is still
mysterious. Experimental evidence from neutron scattering and NMR experiments
indicates that the spin gap might be present only in the bilayer compounds. A
naive calculation for a two plane Heisenberg model locates the order-disorder
transition only for very large exchange coupling between the bilayers. We
propose a interplane relaxation experiment which might quantitatively estimate
the strength of the bilayer coupling. We make detailed predictions for the size
and the temperature dependence of the interplane relaxation rate.
