Also, there's the contrasting nature of their fighting style; Meiling uses martial arts, Reisen uses firearms.
Those are the type of pairings I really like in Touhou. (not just meaning yuri but good friendships also)
Sakuya x Youmu - knives vs swords (haven't seen this pairing in a loooooong time) Reisen x Youmu - guns vs swords (one of the cutest pairings) Sakuya x Meiling - knives vs hand-to-hand (one of the ones that just makes so much sense) and now we have Meiling x Reisen - hand-to-hand vs guns (could be a new fav.)