I don't think you have any chance of that realistically, and you are most likely to end up sat apart. It looks like AB in Economy should be easily available, and of course its very much personal choice, but I don't understand why you would prefer to be AB in a ABC block in Economy where you would have someone in the aisle seat next to you and have to squeeze past a stranger if you wanted to get out, compared with better seats in premium on a long flight, next to each other, with one aisle seat in the middle block where you'd also only have to ask your partner to move?