I generally clip the new plants off the runners after all of them on the runner have acheived 4" to 6" in height and all have multiple leaves. I just use a pair of sissors to cut the runner in between plants. I'm sure they can probably be trimed sooner than I do.
If your plant has two crowns you can split it, separating the two crowns down the middle. To tell if your plant is like this you should be able to see two growth centers where the new leaves emerge from.