That's not exactly "balance", really, as 2x +16 STR materias (or +30 Attack Power) materias are going to be inherently "unbalancing." It's ultimately an economic thing to make the dual materia items super rare.
There's a strong argument that this is too punishing though. After all, to get a good materia you need to burn a good item. Just a quick recap of what it takes to get a 2x materia item, if you're lucky.
Get an item to 100% Spiritbond -> Convert to materia
Get an item to 100% Spiritbond -> Convert to materia
Get a 3rd item
Attach materia 1 (100% success)
Attach materia 2 (25% success)
That means that if you fail on the 2nd attachment you are basically throwing away 3 whole items, and not just 3 made items but 3 items you had to put significant effort to grind spiritbond.
For an expected value of items you can expect to burn to get a 2x materia item: 12. That just seems like a LOT of items and possibly too far on the "rarity" spectrum. For 3x materia: 57.
I don't know, but this seems a little ridiculous to me, and while I do understand that dual materia items are incredibly powerful, it doesn't really make them "balanced" by making them more rare, it just makes them less common.
Looking at the Item prices I've seen on my server for things with 2 materia, 8-20m gil... And I understand why, after all it takes a huge effort to get. But with Gil draining from the economy rapidly, and the fact that even level 50 leves only give a mere 6-10k gil, I'm not sure how the economy can sustain itself in this way for too long.
In essence, there won't be enough gil in the system for items with more than 1 materia to be saleable.