Actually, two megas limit your options.
The reason being is that you lose your item slot + many GREAT megas really suck when they aren't megas (Kanga, Charizard, Mawile, Manectric) or suck if they're built to go mega and don't. With that said, the BEST scenario to use two megas in is when they play vastly different roles on a team that can function properly with 3 pokes and one or the other of them, as you said. However, unlike as you said, it doesn't really expand horizons. It actually limits them a tad and makes people more predictable.
Example: That guy has a Mega Mawile and a Kangaskhan, while I have a Bisharp and a Salamence. He doesn't want to give me free boosts before getting burned by Fire Blast, so he's gonna use Mega Kanga.
or
That guy has Char Y and Mega Mawile. I'm using Trick Room, so OBVIOUSLY he's gonna use Mawile to take advantage of the field effect while it's up.
It's easier to tell who's gonna do what when there's two mega options in the team preview.
It's easy to see if you compare their team to yours real fast. What would they be scared of more? What would YOU be scared of more? Those criteria usually decide it rather easily. And, if there's Kanga, expect Kanga in most cases unless you have like...a crap ton of Intimidate and WoW users.
Nugget Bridge