One could make a 20x20 map with two skyrangers side by side, counting the craft as a "skyranger squadron" to simulate it. In this case, the squadron would always operate together though, which isn't quite the same, of course. The 40k mod already does that with multiple rhinos or drop pods counting as one transport.
But overall, twice as many soldiers and enemies on a map twice as big usually means twice as tedious of a mission. Not an improvement.. Bringing more people to base assaults and big UFOs would make sense though. As an XCom Commander, you can be sure I'd send more than one team to strike bases if I could..
Balance wise, it's hard to argue that two skyrangers full of low tech soldiers is better than an avenger full of high tech ones, and both would often be called to answer the same large threat: terror missions, battleships, alien bases, etc. There could be a list of allowed craft pairs for missions thus allowing modders to define what pairs can land in what missions: You can't justify 2 skyrangers to clean a small scout, or the terror missions of the first few months, but once the council recognizes what terror missions are (Research project that stops "easy terrors", maybe sectoid and floaters only, and starts "real terrors", with snakemen and mutons, they allow bigger deployments of terran assets (ie 2 skyrangers, but not two lightnings/avengers because they don't want to risk two of those super high tech crafts, of could be tied to terror units research projects: Once you encounter chryssalids, they let you bring more firepower).
Similarly, maps scaling to the number of crafts used could make sense: The first craft always lands in the best possible LZ, but the 2nd will have to settle for one further away, so to encompass the whole battle, the map gets bigger and bigger. Civilian numbers should also scale when present (so you send 2 transports to a terror, you cover twice as much city with twice as many civilians and aliens are more spread out). Cydonia would also scale like that: If you send one, they can land as close to the entrance as possible, the 2nd has to land further. And in this case, since it's alien territory, alien numbers would also scale. This gives you the choice of the canonical surgical suicidal strike at Mars, or sending 15 avengers for an all out martian war.
