First, it appears that the counter-rotation (CR) can bring more stability. Even a small fraction of CR stars has a stabilising influence with respect to bar formation (m = 2), since the disk has then more velocity dispersion (Kalnajs 1977). But a one-arm instability is triggered, for a comparable quantities of CR and normal stars. This comes from the two-stream instability in flat disks, similar to that in CR plasmas (Lovelace et al. 1997). There develop two m = 1 modes in the two components, with energies of opposite signs: the negative-E mode can grow by feeding energy in the positive-E mode.
A quasi-stationary one-arm structure forms, and lasts for about 1 to 5 periods (Comins et al. 1997). The structure is first leading, than trailing, and disappears. The formation of massive CR disks in spirals has been studied by Thakar & Ryden (1996, 1998).