I'm currently researching the intermodal distance of transverse and axial modes in optical cavities. But the equation defining the longitudinal separation contains the +- sign, does this imply that both those positions there are longitudinal modes?

Δν_transverse=c/(2∙L) [cos^(-1)⁡(±√g_1g_2)/π]

Furthermore, do the higher-order transverse modes of say; axial mode 1, overlap with the positions of higher order axial modes?