  One of the most important problems of magnetotail dynamics is the substorm
onset and the related instability of the magneotail current sheet. Since the
simplest 2D current sheet configuration with monotonic $B_z$ was proven to be
stable to the tearing mode, the focus of the instability investigation moved to
more specific configurations, e.g. kinetic current sheets with strong transient
ion currents and current sheets with non-monotonic $B_z$ (local $B_z$ minima
or/and peaks). Stability of the latter current sheet configuration has been
studied both within kinetic and fluid approaches, whereas the investigation of
the transient ion effects were limited to kinetic models only. This paper aims
to provide detailed analysis of stability of a multi-fluid current sheet
configuration that mimics current sheets with transient ions. Using the system
with two field-aligned ion flows that mimic the effect of pressure
non-gyrotropy, we construct 1D current sheet with a finite $B_z$. This model
describes well recent findings of very thin intense magnetotail current sheets.
The stability analysis of this two-ion model confirms the stabilizing effect of
finite $B_z$ and shows that the most stable current sheet is the one with
exactly counter-streaming ion flows and zero net flow. Such field-aligned flows
may substitute the contribution of the pressure tensor nongyrotropy to the
stress balance, but cannot overtake the stabilizing effect of $B_z$. Obtained
results are discussed in the context of magnetotail dynamical models and
spacecraft observations.
