In this strange complex of time and space anything can happen

Sorry, I was referring to a production schedule rather than a broadcast schedule. Essentially, instead of breaking your production block down into week-long chunks, at the end of which you're supposed to have finished one episode, you break it into fortnightly chunks, at the end of which you've done two. This allows you to do more scenes that use the same set all at once, rather than recording scenes in script order, as was generally the case in the Hartnell and Troughton eras. So it actually saves time. But it requires a bit more thought and preparation, which didn't really become feasible until Who stopped being on almost all year, and dropped down to only half a year.

In broadcast terms, it was always one episode per week, until it moved to two in the Davison era.