In the energy region 10-100 TeV both the c.r. composition from direct measurements and the cross section for high energy secondary production in the very forward region for p-air interactions are rather uncertain. Contemporaneous measurements of the total energy and of the threshold energy/nucleon of the primary particle can be provided by the atmospheric Cherenkov light and high energy muons. These measurements are performed by the combined operation of the Cherenkov array of the EAS-TOP experiment on the surface (810 gcm2 atmospheric depth) and of the LVD experiment in the underground Gran Sasso Laboratories (3300 m w.e.; Ethμ = 1.3 TeV) leading to the measurement of < Nμ > (Eμ > 1.3 TeV, E0) in the given energy range. The combined operation of the experiments and preliminary results are reported.