I get so floored by the cost of flying that I always search for very low sale fares, and therefore we're usually dealing with nearly sold-out flights and we take what we can get.
My son LOVES to look out the window, so it's better for us to split up so we can have window seats, instead of doing the across the aisle thing. Like TJA says, it's a nice opportunity for one-on-one time, and then we can mix it up as we go.
It is different with twins only because the OP's twins are 2. They both have to have their own seats because they're over the "babes in arms" age, but they're also both still very small and need a lot of close supervision. For us, when we flew with them at 2, we brought their car seats on board. It was kind of a huge pain to buckle them in (we had giant FPSE ones), but it was worth it because they felt very comfortable and familiar in them--it was kind of like bringing a little bit of home on the plane.