Molecular nanomagnets containing 4f ions are attracting an increasing interest because of their large magnetic anisotropy and slow relaxation of magnetization originating from their unquenched orbital angular momentum. Clusters made of such ions are promising candidates for ehancing their slow relaxation of the magnetization at relatively high temperatures. Recently, an interesting family of asymmetric Ln2 dimers has been reported with sizable exchange interaction between the magnetic ions and large single ion anisotropies. Here we propose to exploit the MARI and IRIS spectrometers to investigate the spin dynamics of two compounds of the series, Yb2 and Er2 dimers, to better understand the role of the exchange interaction and crystal field in the relaxation mechanism.