I have found this documented in an official document from 1995 named The Design of Pedestrian Crossings . Specifically (but not limited to) the following items:
5.1.5 Drivers must have a clear view of at least one signal head on approaching, and when stationary at, the stop line. Where the view of
the vehicular signals is reduced by the vertical or horizontal
alignment of the road or other situations such as masking of signals
in heavy traffic conditions or by overhanging trees, the conspicuity
should be enhanced. This can be done, for example, by the provision of
additional secondary heads, tall posts, building out kerb-lines if the
carriageway width is sufficient or installing signals over the
carriageway. Such overhead signals should be considered as
supplementary to and not replacements for those listed as minimum in
the relevant Regulations. If the overhead signal option is to be
considered the problems of maintenance should be taken into account.
And....
5.1.7 To assist blind and partially sighted pedestrians, as they approach the crossing, the primary push button/indicator panel should
normally be located on the right hand side. The alignment should
encourage them to face oncoming vehicles. The centre of the push
button should between 1.0 and 1.1 metres above the footway level.
And...
5.1.8 At Toucan crossings it is normal to install a push button unit either side of the crossing place at the height quoted above. Special
arrangements may be needed if an equestrian push button is required.
Therefore...
The lower one (as mentioned in another comment) is to allow all users, including handicapped ones, to see and interact with controls at their own accessibility level, while the other is to allow all other users have a clear view. Think of a crowd of pedestrians, then both signals: If you had only the lower one, people at the back and drivers wouldn't see the signal. If you had only the higher one, short people on the back wouldn't see the signal either. Thus, you need both of them
Practical Testing
Here's a case that may happen with users with disabilities, handicapped or simply small height (such as children or people with dwarfism conditions). Please note the visual angles (in this example I considered all this people has the widest possible visual angle, around 75 degrees)
As you can see, it would be very difficult for a person in a wheelchair to see the upper sign. Similarly, second and third person in the row won't see the lower sign
