The fundamental vibrational modes of (B2H6)-B-11, (B2D6)-B-11, (B2H6)-B-10, and (B2D6)-B-10 have been obtained from vibrational configuration interaction calculations including up to 6-tuple excitations. An n-mode expansion of the underlying potential energy surface obtained from explicitly correlated coupled cluster theory was truncated after the 4-mode coupling terms, which were found to be very important for these particular systems. All molecules are strongly affected by Fermi resonances, which are discussed in detail. The final results are in very good agreement with the available experimental data.