Before 1956, the NME always listed each side of a double A-side single separately on the chart, with the sole exception of the Rosemary Clooney/Jos Ferrer hit from 1954. This would seem to be a reasonable exception, as both sides were effectively the same song sung from either side of the gender spectrum.
However, in 1956, the NME began listing some double A-sides as joint entries, beginning with Lonnie Donegan's 'Lost John'/'Stewball' for a few weeks of the single's chart run. During the 1956-59 period, the NME's policy remained curiously inconsistent, with some double A-sides listed jointly, and some separately, without there seeming to be any kind of logic behind each decision. I suppose it's possible that record companies might have had an input into deciding whether the NME should list their singles jointly or separately, but that's just a theory.
During 1956-57, there seemed to be a pretty even split between double A-sides listed together and separately, but in 1958, this shifted towards a preference for them to be listed separately, ironically just as the other charts were going the other way. After Elvis Presley's 1959 #1 'A Fool Such As I'/'I Need Your Love Tonight', the NME seem to have adopted a consistent policy of listing double A-sides separately, only making exceptions for a handful of Beatles double A-sides in the 60s which were apparently being requested as "the new Beatles single" in record shops. It's not known exactly when the NME finally switched to listing double A-sides jointly, but they were still listing them separately as late as 1967.
In 1956, the NME seemed to be leading the way where listing double A-sides jointly was concerned, with Record Mirror and Melody Maker both consistently listing them separately. With (I think) one exception, RM didn't list double A-sides jointly until mid-1957, when Lonnie Donegan's 'Gamblin' Man'/'Putting On The Style' became the first double A-side to be listed jointly in both NME and RM. This signalled a permanent change in policy for RM (although curiously, RM deviated from this policy on one occasion, listing each side of Cliff Richard's 1960 double A-side 'Fall In Love With You' and 'Willie And The Hand Jive' separately).
During the latter half of 1957, only one double A-side was listed jointly in RM without also being listed jointly in the NME, this was Elvis Presley's 'Lawdy Miss Clawdy'/'Trying To Get To You'. However, in early 1958, two very big double A-side hits had each side listed separately in the NME chart, Perry Como's 'Magic Moments'/'Catch A Falling Star' and Pat Boone's 'A Wonderful Time Up There'/'It's Too Soon To Know'. Soon the floodgates were open. Around the same time, in March 1958, Melody Maker changed its policy of splitting double A-sides, with no exceptions, to one of listing them jointly, with no exceptions.
The newly launched Disc chart suffered the same kind of inconsistency as the NME for the first couple of months, but soon settled down to listing all double A-sides jointly.
I've had an idea that I might change my plans for the definitive chart thread, leaving it as it is up to the beginning of March 1958, but adapting the NME chart so that all double A-sides are listed jointly starting from 08.03.58, the week that Perry Como's 'Catch A Falling Star' first appears on the NME chart, which seems to be the crucial moment when the NME breaks away from the other charts and goes its own way on the issue of double A-sides, and just a week after MM started listing double A-sides jointly.
Perhaps this would be the right time to start incorporating data from all four charts, as it was also the same month that the BBC chart began, which did the same. This would mean that The Everly Brothers would get a #1 with 'Bird Dog' after all!
There are potential problems, of course, most obviously that listing double A-sides jointly when they're separate in the NME would free up positions at the bottom of the chart. These could often be filled by singles which only appeared in charts other than the NME, but sometimes the #30 or even #29 would have to be guesstimated based on the highest placed dropout from the previous week.
The other potential problem would be the 1959 newspaper strike, as with no references for positions for jointly listed double A-sides, the system would come crashing down. However, the BBC charts exist in full from the strike period, and these would be a perfectly acceptable substitute for the three missing charts.
I'd like to get some opinions on this approach. I would of course start a new thread listing the untouched NME charts beginning 08.03.58.