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?

$Δν_{transverse}=\frac{c}{(2L)} \left[ \frac{cos^{-1}⁡(\pm \sqrt{g_1g_2})}{π} \right]$

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