I've hit the stage where the intarwebz are seriously interfering with my nanocount, so I'll go all meta and write about writing (write about not-writing?) instead of writing.
Started by dipping back into some source material, picking up Evelina (by Fanny Burney), and on the Richardson side, Pamela and a (severely trimmed so that it fits into one volume) Clarissa Harlowe. I've read a severely-trimmed Sir Charles Grandison, with lovely Chris Hammond illustrations, but this is my first acquaintance with Clarissa Harlowe. That for the novel of sentiment/manners, trusting that I had Jane Austen's works reasonably well absorbed into the hindbrain, having read them all twice (okay, except for Mansfield Park, that only once) as well as most of the juvenilia and several continuations-by-other-hands.
For the Gothick, I had Clermont, Castle of Wolfenbach, Manfrone or the One-handed Monk, and The Passions by Rosa Matilda, having read the first two previously, and dipping into the second two. There's a beautiful passage from Manfrone that I will try to quote later.
For scholarly material, I had The Epistolary Novel in the Late Eighteenth Century, by Frank Gee Black and The Gothic Novel 1790-1830, by Ann B. Tracy, which is a collection of plot summaries (all hail Ann Tracy!) and index of motifs that makes for somewhat hilarious reading--one of the reasons synopses are so difficult is that summary piles on what narrative portions out, and the effect can be, um, bathos instead of pathos--as Tracy admits.
Back when I'd come up with the original concept, I'd thought of the two correspondents as being cousins, and each writing from her own coign or eyrie (look, it's already affected my vocabulary) of genre and convention. When I came to the point of needing a plot, what swam up from the depths was the old twins switching places plot (because the idea is not to be original), allowing for more explanation and observation and complication all around.
Which meant not starting in media res with an existing correspondance, but bringing the two to the same place so that they could become acquainted (because obviously they had been parted and kept in ignorance of each other, I mean, obviously!) and the switch could be effected.
Which meant they had to start out with other correspondents to whom to confide their situations, hopes and fears, and thus I had more characters all at once. As to be expected with twins, there was immediate mirroring of their circumstances. Each had an older woman companion or mentor (with her own secrets), and each had an older man who stood in as grandfather. Each was about to be removed from her home and sent to the place (London) where she could meet her long-lost twin.
Ethelinda is the good twin. As you might guess from the name, she and her circs are a hommage (as they say in high-toned literary circles, rather than copy or rip-off) of Evelina, including her fluttery old hen of a clergyman guardian (I heartily disliked Evelina's guardian, so I'm taking some resentment out on his double), but for female mentor I've given her a more redoubtable sort, Lady Fortuna Beldam (I'm so in love with that name you wouldn't believe) based on early women travellers like Lady Mary Wortley Montagu and Lady Hester Stanhope.
Poking through my background reading, I realised that the divide between the Novel of Sentiment/Manners and the Novel of Gothick Horror (they weren't called Gothics in the day, any more than Gothic architecture was called that in the 14th c.) was much less a chasm than a ditch. The highly-coloured and unlikely events of the Gothick, the abductions, the imprisonments, the forced marriages and secret marriages, the concealed births and disinherited heirs ... pretty well all happened in novels of sentiment as well. Richardson built his reputation on Pamela, a long series of abductions and attempted seductions, but of a servant girl instead of a heiress (Clarissa and Sir Charles Grandison are about gentryfolk abducting each other). Evelina, because of a secret marriage and concealed documents, is really the heir to two fortunes, but she has been raised in rustic seclusion, just as Ann Radcliffe's heroine was in Mysteries of Udolpho.
Evidently I would need to distinguish the two storylines by something more than choice of events.