Two branches of Solutions are:
1) k(L1+L2)= 2m*pi + sin^(-1)( \sqrt(r) * cos k(L2-L1)) - sin^(-1)(\sqrt(r)),
and
2) k(L1+L2)= (2m+1)*pi - sin^(-1)( \sqrt(r) * cos k(L2-L1)) - sin^(-1)(\sqrt(r)).
For more detail please check the Eqs.(10)-(12) in the following paper:
W. J. Fader, Theory of two coupled lasers, IEEE J. Quantum Electron. 21, 1838 (1985).