My go-to and really favorite double battle Pokemon in anything is Charizard and Swampert. After the first time I went onto Pokemon Showdown and I was wanting to make a team I went to the team maker screen and I had to think about it for a bit until I got it. To this day I have loved having them together in double battles because of these reasons:
This Pokemon can vary in it's stats depending on what you raise it to be, fast and physically strong, bulky and is a great special hitter, etc. It has a good typing on it which is Fire/Flying, while it is hating those Rock types, still gives you a good advantage in game and other things. It has a wide arrange of attacks that it can learn that is great for it's teammate such as Sky Drop where it can take a Pokemon off the field with it, into the air, then on the next turn it drops it to the ground and it takes the damage; I find this a way better move than Fly because it does more even if it's weaker.
In double battles or even triple battles this is what I find the best move to have when pairing with Swampert because if there is a Pokemon with a move you think can do good damage on it and Charizard is faster than it, it is off the field, it ain't doin shit to Swampert, go ahead and Rock Slide or Earthquake.
Such a good balance between earth and sky is with these two. Both starters of an odd numbered gen, both pretty popular in the fandom, both unique in typing for starters, yet really different, and they can learn such useful moves. Swampert is also like Charizard in that it can be raised in multiple ways for fights, it is perfect for Charizard as a partner because it is slower than it and because of that it can use a move like Surf and the faster Charizard can use Dig or Sky Drop to get the hell outta thereya! Again because of Charizard being a Flying type when it comes down or up use EQ and hit everyone except you team mate.
Now if you're going for a triple battle team with them then I would say use a Levitate user that can heal Swampert because that Poke will likely be eatin up those hits without a way to restore itself. And with the advent of Mega Evolution and they both have one it makes the team build even more fun and thought building as to who do you want to Mega Evo and don't and what will they have for that.
Well that's all for now and until next time, wubba lubba dub dub!