My understanding is that you must have both locators transmitting but need only one ADF receiving to do the approach.
Once established over the 1st aid then it has done its job ie position the a/c within a defined area prior to commencing the next segment of the approach. Thereafter the 2nd aid is acceptable to be used for tracking to stay within the required tolerances.
Playing devils advocate here, but if both receivers are required at what point is the changeover specified for tracking from the locator behind, to tracking to the locator ahead?
It's easy to say 'at the mid-point' but for a TWIN LOC only approach ie no DME, this doesn't seem to provide a means to determine this. Timing guidance isn't given for the segment between the aids (haven't got an approach plate with me so must rely on memory).
Given the different tolerances different receivers provide there is also a possible tracking discrepancy on the changeover between the two.
I realise this is somewhat pedantic given that it's a non-precision approach, but just my thoughts. Using two ADFs then I have one tuned in front & one behind as this saves twiddling the buttons during the approach. After the 1st LOC I track using the ADF tuned to the beacon ahead.
I find the one behind is useful once overhead that beacon for determining if I'm L or R of the track by the way the needle falls (slow, fast, to the L or R etc). During inbound to beacon #2 then the combination is great for determining L or R of the track quickly. Even so I still track using the navaid ahead.