Which clock is really slower? (Not the Twin Paradox)?

As far as I know, it is only possible to compare the clocks and find the slow clock after the ship slows down and goes back to mee the other ship. If the two ships keep moving relative to each other, both will see the other ship's clock as moving slow and this cannot be reconciled without one of the ships accelerating in some direction in order to match velocities. I'm no expert on the subject, though, so I could be wrong.