How difficult would it be to make this?

That sounds pretty interesting.

Though I have to ask.

Would it be a 2 player vs 2 player match?

IE:
One person control the cow while the other person control the player on the cow?

If you can put a man on a Redeemer and fly around on it I am pretty sure it can be done.

It should be accomplishable, but may require some custom pawn code to get around hacky solutions... though as Skillz mentions the Strangelove mod worked pretty well soo...maybe using that as a base (for a good example of "attaching" players to other pawns).

I'm thinking the only real trick would be to grab the controlling player's input (the strangelove mod might behave a tad different from what you would want). Grabbing the player pawn's acceleration vector should work though.

I remember doing this a long time ago. The idea was to make a cow work like yoshi, and be able to ride and guide it. One of the several problems that didnt allow pawns to connect was a script that forces a pawn to automatically jump off another pawn.

My guess on something like this would be swapping a PlayerPawn class that can ride the cow without conflicts when both connect. Doing something like this might have conflicts with other mods that replace the playerpawn class though.

You get noticeable lag and stutter when the playerpawn collision boxes bump and touch each other. Ask any MH player, they see that all the time when a hunting group runs down a corridor together. It was so pronounced I just removed players from colliding in MH and that cleared it up though the passing-through of players is a bit odd until you learn to ignore it.
My guess is that you'd need to code something akin to 'magnetizing' the players. Cows would be positive, jousters negative (hang on, I'll explain). Normal player collision rules would apply until a cow and a jouster from the same team encroached on each other. When that happens the jouster would lose mechanical control of his movement (like he's on the SLV rocket) but still be able to rotate his orientation (look around). The cow would be fully in control of the movement for both players. Finally the jouster would be locked to a designated bone point of the cow (mounted) until he either ejects or gets killed. Personally I'd not allow ejecting and force the pair until someone dies.

Finally I'd make the weapons only damage on a headshot on the jouster. This will prevent cow killing and force the teams to go after the same point on each other. This would also allow some defensive measures too since your lance (or whatever) will already be held high. You could even do mounted swordfighting pretty easily once you got to here.

....
Yeah, I just watched Heath Ledger's movie. Fuck me, right?

Gotta have cow killing. The mounted guy still lashed to the dead beast, twisting on the ground, able to fire but unable to retrieve ammo, powerups, etc, until someone kills him as well. If it is the same guy who killed the cow, double kill!

Or if the cow is killed, mounted guy is thrown off and vice versa.

If however you cannot kill the cows and they too are armed (will they be armed? Killable and unarmed is just cannon fodder) then they are gods. If you can't kill them and they are unarmed then who the heck would want to play a battle horse simulator?!