The nature of the spectrum should be obtained by comparing tunneling coefficients.
But, in the case of two interacting bands, we have two sequences of intervals containing
spectrum.


Hence, there is a possibility of resonances between intervals from different sequences.

The usual double well problem:

consider the matrix , its eigenvalues are  where 

In the present case, the distance separating these intervals plays the role of 
The tunneling effect, , relevant here is the horizontal tunneling  associated to