This can be done both online, or through the co-op campaign.
For the co-op campaign, you can play online with someone else against another team of two players or play the co-op campaign locally with two controllers.
Addendum by thatkidfromohio
You can set this up under player match online and if you both have an extra controller you can boost it.
So there you go, it IS available online according to one user, just have to jump through some hoops (extra controllers) to get it.