An unusually well-preserved diptych dated 198 CE in the Bodleian Library, Oxford, shows how this document format was used for legal documents during the Roman empire.
"The diptych contains the appointment of a guardian for a woman by the prefect of Egypt. The main body of the text inscribed on the wax is in Latin, followed by a subscription written in Greek by an amanuensis on behalf of the woman, who was illiterate. On the outside there are copies of these sections and a list of the names of seven witnesses, all written in ink directly on the wood. The diptych was originally tied shut and sealed with the seals of the witnesses to prevent tampering with the inner text, the authenticated version, while the exterior text remained available for consultation" (Hunt, R.W., The Survival of the Classics, Oxford: Bodleian Library, 1975, no. 32.)
Looking for further information on this diptych, I found a full description by B.P. Grenfell entitled A Latin-Greek Diptych of A.D. 198 in The Bodleian Quarterly Record, 2 (1920) 258-262, from which I quote:
"An admirably preserved Latin diptych with Greek signatures was recently acquired by Professor A. H. Sayce in Cairo and presented by him to the Bodleian (Lat. inscr. 10-11). The two tablets A and B, measure 15 x 12 cm., and are of wood, with holes at the usual places each having on the inside a sunk wax surface measuring 10.5 x 7.4 cm. The interior (pp. 2-3) is inscribed in Latin cursive with an official authorization from the praefect of Egypt on September 23, 198, fo the appointment of a guardian (tutor) for a woman called Mevia Dionysarion, whose signature, written for her in incorrect Greek, is appended. The exterior B (p.4) contains in Latin (1) the names of the seven witnesses customary in Latin documents of this kind, (2) at right angles to the other writing, the beginning of the usual duplicate of the main text. The exterior of A (p. 1) as (1) the end of the duplicate with the signature, (2) the title in Greek. The... diagrams show the arrangement.
"On the exterior of B there was originally a narrow band of wax covering the string which ran through the two central (and larger) holes, and on this the seals of the witnesses were impressed. The impressions have perished, but traces of the indentation left by the string on the wax are preserved.
"The arrangement of the exterior writing in our diptych is the same as that found in two single tablets from Egypt which originally formed parts of Latin diptychs. One of these (Lefebvre, Bull.soc.archéol. d'Alex. xii.39 with plates) is a military diploma of A.D. 94 discovered in the Fayûm, and corresponds to our B, except that wax was not used on the interior; the other is a fragment which I obtained in Cairo and is in the Bodleian (MS. Lat. class.e. 16 (P.); De Ricci, Proc. Soc. Bibl. Arch. xxvi. 195). It is dated A.D. 143, and though the nature of the contents is uncertain, the general arrangement clearly corresponded to that of our A. Evidentaly therefore the order of the exterior writing on our tablet was not uncommon, though the customary arrangement of the exterior writing on a diptych (e.g. Bruns, Fontes, Juris Rom. 98) is just the converse, the list of witnesses being placed on p.1 and the duplicate of the main text on p. 4. Since in the present case the duplicate was too long to be written all on one side, the arrangement adopted may well have been due to the influence of the customary arrangement of a triptych (e.g. Bruns, op. cit. 130), according to which the main text was on pp. 2-3, the list of witnesses on p. 4, the duplicate on p. 5, and pp. 1 and 6 were blank. By tranferring the duplicate to p. 1 the third tablet could be dispensed with, and a desire for economy seems to have led to the rather awkward arrangement of the exterior writing found in our diptych and its parallels...."