  The concept of isochronous mass spectrometry (IMS) applying two
time-of-flight (TOF) detectors originated many years ago at GSI. However, the
corresponding method for data analysis has never been discussed in detail.
Recently, two TOF detectors have been installed at CSRe and the new working
mode of the ring is under test. In this paper, a data analysis method for this
mode is introduced and tested with a series of simulations. The results show
that the new IMS method can significantly improve mass resolving power via the
additional velocity information of stored ions. This improvement is especially
important for nuclides with Lorentz factor $\gamma$-value far away from the
transition point $\gamma _t$ of the storage ring CSRe.
