Cloning is definitely a thing in the Nanoha universe; I think the parts of Project F that had it be illegal were just the memory-transfer (to "duplicate" deceased individuals) and the Artificial Mage process to supercharge their magical capacity; I don't know if reproductive cloning is allowable or just theraputic cloning (replacement body parts). That said, we do have a childless couple canonically adopting two clones of the wife (and after the wife died, the dad adopted yet another clone of his wife along with three other teenage girls, at least one of whom was in the strike force that killed his wife; nobody's got a bigger heart than Genya Nakajima). Another possibility would be creating familiars, like Arf or the Liese twins; at least some of those are treated like the children of the creator, though their legal status as independent beings is a little sketchy, and they wouldn't survive the death of the creating mage, so it's kind of an ethically dubious idea. (Linith, Presea's familiar, was created from her pet cat specifically to raise and train Fate and died when that contract ran out, which, nauseatingly enough, seems to be the norm; Arf was created to save a dying wolf cub's life and Fate basically gave her complete freedom as soon as she'd realized what was going on.)
Honestly, the fact that they're both orphans might lead them more towards the Vivio route of adopting, though I get that this is the punchline.