Maybe, but unlikely.

2M is considered the low end for the possibility of black hole formation. But for the resultant mass to be exactly 2M, you'd have to have both bodies' motion perfectly cancel each other - a glancing blow by either body would result in some of the mass being ejected from the system. And at a fraction of c, the fractional mass would quickly escape the larger mass's gravitational influence, so it wouldn't return and reform. The odds of two bodies having the exact opposite x, y, and z velocities is just, well, astronomical.