It requires some housruling and decisions by the DM, mainly with the whole needing to act simultaneously on initiative thing. My friend and I did this once, and it was amusing while the game lasted. I was a Human Knight/Paladin working towards Cavalier. He was a Centaur starting out in Knight to also eventually go into Cavalier. Our dual lance charge could pretty much vaporize anything, I could protect him with the Ride skill, and I could drop down and use him as cover when I needed it, as well as being able to Shield Other each other.
As I said, pretty fun. Probably the most powerful sheer melee combat can be, aside from Tome of Battle (I WAS gonna dip Warblade...), but considering we had a Kineticist psion in the party, it wasn't like we were breaking the game.