How about this? One of the most wholesome books of recent years – well, before its fans got to it, it was – and one of the least wholesome. I love it.
The lynchpin that connects these two apparently dissonant worlds is the existence of Nicholas Flamel in both of them. It’s worth noting at this point that Flamel was a genuine historical alchemist, and one frequently rumoured to have obtained immortality by creating the legendary Philosopher’s Stone – both sources start from the historical Flamel and extrapolate him from what little is known.
Fortunately there are a great many parallels between the two settings. Both pre-suppose that the world is weirder than most normal people ever realise, and both accept the existence of magic. Handily, both define their magic in fairly general terms, which allows GMs a lot of latitude.
Al Amarja already exists as a secret enclave unknown to most of the world – while it is improbable that Hogwarts itself exists there, it is very likely that a magical college of some description does. How exactly Her Exaltedness regulates this is a difficult question. Most likely, the Al Amarjan Ministry of Magic is heavily infiltrated by the Movers of the Hermetic Cell. This would allow the D’Aubbaines to control it – as much as they control anything else – through their own Mover connections. The Edge itself would be not unlike the village of Hogsmeade, an open zone where wizards may use magic freely and without regard for Muggle sensibilities. It’s worth keeping in mind that the society of wizards and witches, and attendant magical creatures, exists all over the world, too, however hidden. GM’s who are fond of the Neutralizers will find plenty to keep them busy in such a world.
Integrating the two settings is probably best done on a character by character basis, rather than through the various factions.
Harry and his friends, for the most part, should probably be kept as NPC’s, and player characters should be created as graduate wizards. This will require characters who are considerably more powerful than standard Over The Edge characters – which means many other characters native to that setting will need beefing up to keep up, especially the major antagonist characters.
Nicholas Flamel can be used pretty much exactly as he appears in At Your Service – assume that he is merely a little more duplicitous than he was portrayed in Harry Potter and the Philoshopher’s Stone and it should all fit pretty well.
More fun are the characters already resident in the settings – did Sir Arthur Compton take his first steps in the Dark Arts as a member of Slytherin House? Will George and Fred Weasley join the Cut-Ups after their graduation for Hogwarts? Why does Islam Petrie know so much about magic if he’s just a Muggle? Is Al Amarja where Sirius Black hides out from the Dementors, and if so, who helps him evade them? (He’s almost certainly a resident of Chateau Melmoth if he is on the island.) Do the Agaras seek the assistance of wizards who are Animagi or skilled in Transfiguration to help them with their condition? And just where does Snape buy those nasty ingredients, or Hagrid find those incredible creatures?