We present the study on continuous high-resolution spectroscopic observations of two long-period single-lined RS Canum Venaticorum (RS CVn) binary stars IM Pegasi (IM Peg) and σ Geminorum (σ Gem), obtained with the Hertzsprung SONG telescope during the 2015-2016 season. Chromospheric activity indicators Hα, $\rm{Na\,\,{\small I}}$ D1, D2 doublet, $\rm{He\,\,{\small I}}$ D3, and Hβ lines have been analysed by using the spectral subtraction technique. The expected chromospheric emission features in the Hα, $\rm{Na\,\,{\small I}}$ D1, D2 doublet, and Hβ lines confirm that both of two stars are very active systems. In the spectra, the $\rm{He\,\,{\small I}}$ D3 line had been always detected in absorption feature. Although the behaviour of chromospheric activity indicators is very similar for both stars, the activity level of IM Peg is much stronger than that of σ Gem. Moreover, the equivalent width variations of the Hα, $\rm{He\,\,{\small I}}$ D3, and Hβ line subtractions correlate well and show different behaviour among different orbital cycles, which indicates the presence and evolution of activity longitudes over the surface of two stars. Furthermore, the subtracted Hα line profile is usually asymmetric. The red-shifted excess absorption features could be interpreted as a strong down-flow of cool absorbing material, while the blue-shifted emission component is probably caused by up-flow of hot materials through microflare events.