Cis-1,3-di-tert-butylcyclohexane can exist in a chair conformation (A) in which both tert-butyl groups are equatorial.  Though, in either chair conformation of trans-1,3-di-tert-butylcyclohexane (B), there is an axial tert-butyl group. The axial tert-butyl group can be avoided if compound B exists in a twist-boat conformation C. in which both tert-butyl groups are equatorial. Evidently the twist-boat conformation is more stable than either chair conformation having an axial tert-butyl group. The interaction of the methyl groups with the ring hydrogens should be about the similar in both conformations A and C, so that these cancel in the comparison.