timew split <id> ...
Ѕplits an interval into two equally sized adjacent intervals, having the same tags. Using the 'summary' command, and specifying the ':ids' hint shows interval IDs. Using the right ID, you can identify an interval to split.
For example, show the IDs:
$ timew summary :week :ids
Then having selected '@2' as the interval you wish to split:
$ timew split @2
timew-join(1), timew-lengthen(1), timew-shorten(1)