I saw a matched pair of 1967 specimens sell on eBay in June-July this year, just before the new Charlton's came out. As I recall the pair sold for $550-$600 USD.
Judging from the book values in Charlton's for the Multicolour and Birds specimen notes, the book value of the individual notes added together is more than book value for a complete set. I haven't done the calculations for the 1954 and prior series specimen sets, but the result is probably the same. While the 1967 specimens are currently priced at $350 each, I don't believe someone would pay $350 for one note without the other; they would expect to get a matched pair for $700.
For the 1967 specimens, while I might buy the serial numbered note alone, I would not buy the double-dated note without the corresponding serial numbered note. It wouldn't be too difficult for an enterprising crook to take a genuine double-dated 1967 $1 bill and doctor it with convincing "Specimen" overstamps and perforations. However it would be a lot harder to convert a regular-issue 1967 note with an I/P serial number into a specimen with the serial I/P 0000000.
Then again, someone with only a genuine serial numbered specimen could "manufacture" a double-dated specimen to go with the serial numbered one. In any event, in my opinion a matched pair of 1967 specimens should be worth more than the sum of the value for each note.