I don't actually know the history of the rule, nor do I have access to a copy of service flying regulations at present.
But, I think that there's various interpretations - truckie TPs being grumpy at only logging PiC half their flights as qualified TPs when the FJ TPs do on every flight, as you say it's quite likely that often one TP is i/c the aeroplane, whilst the other is i/c the testing (although when I was an FTE i/c the testing, I just logged FTO!). Or it may just be some historical view that TPs are skygods who are effectively PiC on every flight, so important is their role. Another possibility is that it's a deliberate compensation for the usually very low hours that most full-time TPs actually get to fly, compared to the hours they work. A colleague of mine recently estimated that he flies around 15% of the hours he's at work - compared to, say, an airline pilot where it's probably nearer 40-50% - although a fighter pilot is probably also down around the 15% figure.
My recollection of the rule (from nearly 20 years ago when I was last subject to a military Flying Order Book) is that it was worded along the lines of "when two qualified Test Pilots fly together for the purposes of assessing an aircraft, whilst the constituted captain will be entered as such in both pilots' logbooks, both shall log P1".
It's consistent(ish) with some other environments - the Space Shuttle never had pilot and co-pilot, it had commander and pilot, and for that matter the Army Air Corps operates the Lynx that way - both commander and pilot are qualified pilots, but their roles are different to what we might understand as PiC and FO in, say, the airline world. Another peculiarity might be the Nimrod, where sometimes the Captain was an AEO or Navigator, because of the mission role - but I'll bet that the chap sat front left still put the duty as P1 in his logbook.
I can't honestly say it troubles me - I solve it in my logbook by just having relabelled a column "test pilot", and keeping a separate tally of that. I can see why it troubles Dave - but equally I wonder when aviation journalists say fly for magazine write-ups, what column they put that flying in?, when there's usually a safety pilot who isn't an instructor - so an argument could be made that the journalist shouldn't be logging it at-all: but I'll bet they do. And, given that said aviation journalist is actively doing a job of work in that cockpit, using their particular skills, the moral argument that they should be logging it as something is strong, even if the legal argument isn't.
G