Optical transmission from hybrid surface plasmon-polaritons (SPPs) in separated metallic double layers is investigated. SPPs in metallic double layers have been understood as that they have the external-SPP mode and the internal-SPP mode, existing with these SPP modes decoupled from each other. However, here we propose that such SPPs can be coupled together by theoretical analysis in double metallic flat layers. Moreover, metallic double layers with nanostructures of deep nanodimple arrays can have hybrid SPP modes from interaction between the external-SPP even mode and the internal-SPP odd mode, owing to quasi-localized SPPs at dimple dips. The numerical simulations and experiments show that these hybrid SPP modes produce enhancement of optical transmission.