Per default universes are assumed to have separate three-dimensional spaces, but should a piece of fiction demonstrate the opposite, destruction of several timelines connected in such a way would only be Low 2-C (Universe level+).
Second, is the case of timelines that at certain points are connected. Contrary to the case where one can always travel from one universe to the other via three-dimensional movement, it is only possible in those at certain times. In fact, at certain points in time they might be the same universe. E.g. if a timeline branches into two, then the timelines were the same universe before the branch split happened. Other way around, if two timelines get merged into one, then they are the same universe only after they were fused.
In those cases, the destruction of any one timeline is only counted if it was not connected to any other timeline for an infinite amount of time. Otherwise, if there are several timelines none of which are separate for an infinite amount of time, they would all be counted as just one timeline for the purpose of Tiering their destruction or creation.