Multi monitor LoL isn't possible because multi-monitor LoL would be cheating. MOBA games must limit the total viewable zone for all players to exactly the same size, and for that reason, they also have a very limited maximum zoom-out.
Because getting a bigger tactical overview would give player an immense advantage in LoL. So if you ever did manage to get LoL to play on multiple monitors with wider view, you'd be banned in a very short order for obvious cheating.
Yeah, this is why I'm thinking it's best to go for a 'one big, two small' setup, to best accommodate games like LoL.