In over 300 games of Dota 2, I have yet to see a dual lane mid. I've seen a few competitively, but it seems the default for matchmaking games is 2-1-2, 2-1-1+J or the occasional trilane. Dual lane mid makes a lot of sense to me, since it allows one hero to grab runes and/or go ganking while the other stays to farm/gain solo XP.
The biggest reason I can think of is because the mid lane is relatively safe from ganks, and that's where you'll want to position your most important character so they level up the fastest in the most safety. So here is no reason to take xp away from them by doubling up.
Also, taking a hero away from either the top/bot lanes makes them much more vulnerable as those lanes are much longer and more prone to ganks. The jungle lane is very important to hold as it keeps your jungler safe (as Lyrion pointed out in his comments) and the top lane is very prone to ganks.
2 Mid is rarely a good choice because of many points:
a) 2 Mid forces a sololane without any(or few) benefits because you could take a jungler into the team which forces a sololane too but provides more gold and xp for all characters.
b) Sololanes (except mid) are not easy and many times doomed to play defensive - only leeching xp and getting very few lasthits - making this lane probably lose vs 2. On the same side: if the soloer does not play defensive he is killed more easily because the sidelanes are much longer providing the ability to chase which is not given in mid which leads to
c) 2 mid dont have much time to chase since mid is so short resulting in a safer soloer which itself results into more xp+gold for the midsolo vs sidesolo.
the only reason 2 mid is sometimes seen is because an enemy team picked such a strong mid that he isnt allowed to get any farm. this enemy hero(+player) is so strong/dangerous that they give up the benefits stated above (example tinker on "X!!!" [playername])
Dual mid requires a team built around it, which is typically difficult in a public game. You want your lane mid to typically be a support hero and a powerful ganker, such as Crystal Maiden+Night Stalker, but this requires you to 1v2 solo in another lane, which not many heroes in the game can do effectively. If you haven't chosen one of those heroes (Windrunner, Mirana, Puck, QoP, and Dark Seer being the most common) then your lanes are usually pretty terrible.
Also, players who solo mid have a high chance of taking over the game as they usually pick snowball heroes. In a public game, this is a popular way for players who feel they are underrated to improve themselves.