Did the sword cause the gem-arm? If it did, I would only say 1 attunement for both. Otherwise, I may let it go with no attunement and give some other boons to the other PCs, like what
@DeviousQuail stated.
This is actually where I think I was heading as I wrote the description above. A powerful religious artifact had been stolen by an unknown faction, and he had proven himself worthy. As both recognition and to equip him for the challenges ahead, he was granted the sword and arm. (Losing the arm was his final test, the willingness to sacrifice himself for the greater good.)
Nobody else would ever be able to use it, even if it were severed. The sword and hand will grow more powerful over time as he does.
I...don't give out a lot of magical items. As of the latest scenario, they had managed to acquire a relatively small sum of cash, enough to buy some healing potions and to live on. On the other hand, they now have a fairly powerful organization somewhat in their debt that they can call on for support. The only magic item they'd acquired was a wand of sleep.
Also, I've nearly killed the group on a couple of different occasions. More my fault than theirs, I'm still dialing in my understanding of power levels in designing encounters.
After all that's gone before, the others have joined the Paladin in agreeing that the artifact needs to be recovered. They've proved themselves as well and also received items to help them along the way.
More importantly...now that they have these items, I have a bit of a buffer in designing encounters and I'm less likely to misjudge badly enough to wipe them out by accident.
All that being said, I like the idea of treating the sword and arm as a single item for attunement purposes, and I think I'll go that route. (Everyone else also received two items...one that requires attunement and one that does not.)