The new Champs are going to be a balancing nightmare
Here is what's going to happen. The new Champs are obviously going to be much stronger if you can play them together. They're going to have to be balanced around this fact. They'll be garbage when apart because of this. If they aren't balanced around being played together, they'll be absurdly OP when they are.
They'll live in this awkward spot where if they're ever strong together, one of them will just get banned every game. There will be no point playing one without the other, so they'll never get picked. If they are strong apart, they'll be completely broken when together. The community will demand Riot nerfs them.
This kind of design seems pretty doomed from the start. Just my opinion though.