For decades, multiaxial fatigue tests have been proposed in order to derive models for fatigue life prediction of elastomers. Nevertheless, the importance of quantifying the multiaxiality has not sufficiently been discussed. In this paper, a new method is proposed to handle the multiaxiality in elastomers fatigue testing. It is based on the pair (K2, K3), two invariants of the Hencky (true) strain tensor. Thanks to the example of the simultaneous uniaxial tension-torsion fatigue of a Styrene Butadiene Rubber, we demonstrate that it is possible to prescribe a constant multiaxiality level throughout different tests. The method consists in fixing the multiaxiality indicator K3, and then varying the strain level with the intensity of distorsion indicator K2. Results show that the fatigue lives seem to unify with K2 regardless of the value of K3. They also demonstrate that the initiation crack angle depends on the multiaxiality indicator K3 but not on K2. Finally, the relevance of the approach is acknowledged by comparing the pair (K2, K3) to the classical largest principal stretch ratio and biaxiality factor.