There was never a requirement for them to be placed at specific minimum spacings. There were spacings recommended in the guidance documents, but the regulations only required them to be placed 'at regular intervals' and that was later amended to remove the word 'regular'. The latest regulations removes any compulsion to provide a repeater sign, leaving it up to the local authority to decide, again subject to guidance, how many are appropriate and where.
The point is that is no longer a requirement. A single sign is now sufficient in either direction and they could be back to back on a single post. However, I wonder whether this is really to avoid a speeding prosecution for speeding failing if one of two signs is missing or obscured.