  A new event mixing constraint, namely invariant-mass/energy hierarchy
correspondence (IMEHC) cut, is introduced for the low-multiplicity event mixing
technique for the purpose of measuring Bose-Einstein correlations (BEC) in
exclusive reactions with $\pi \pi X$ final state particles.The mixing cut is
relevant to the hierarchy of the invariant mass of $\pi X$ system and two
bosons energy hierarchy. Numerical tests are performed to check the validity of
the new mixing method. As long as the measurements of BEC parameters $r_0$ and
$\lambda_2$ are considered, this new mixing method is effective to observe BEC
effects and the systematic bias of $r_0$ and $\lambda_2$ are smaller than the
previously proposed mixing cut.
