Driving each wheel separately certainly could be done, but, there would need to be a serious plan if you wanted it to really be worthwhile. Even then, I gotta ask what "worth" one might actually get if the goal is simply to power the existing two rear wheels (I realize you hinted at perhaps more).
Some of the concerns I would have would come down to adequate speed control methods. Obviously, if you are driving along on a non-slippery surface with one wheel attempting to turn at a higher rpm without a differential involved, your going to feel it in the steering. I assure you that your differential spiders turn more than a guy would think under most conditions. Even small psi differences in the two tires will cause this.
They make these units not too far from me
http://www.millerstn.com/products/nitro-4000-series.php
My neighbor works there and I know these are driven by 4 individual wheel motors. I also know that the hydraulic control of the motors is rather sophisticated to keep the machine stable on the road at higher speeds and also control wheel spin in the fields. Not sure a fellow would have room to fully accomplish something that would be successful if you wanted to maintain similar speeds to what a typical case travels at currently.
Mechanically it wouldn't be too difficult, cast away the original axle, pick some properly sized wheel motors and get them mounted up. I dont think this would work if you did it with used components. I think you would have to have brand new matching motors to avoid as much mismatched speed differences as possible. We all know how different existing used Case drive motors act after they have some years on them. Same goes here.
Of course you would also want to do the math to see if your going to forego any "gearbox", which if you do, you would have to make a compromise between capability and existing horsepower. Essentially, the currently available horsepower is why the tractors have a two speed axle. I bet you would need at minimum, double the horsepower they use now to cover the same range of capability if you were to direct drive the wheels, and I am not counting any PTO usage. I'm guessing here, but I dont think I'm dreaming in technicolor here.
I like the idea of two drive motors, but only in the realm of creating a small Case GT sized example of a crawler where travel speeds verses traction become a much more controllable experiment. The results of that would be as predictable as the many small track based trenchers that have been successfully used over the years.
If your main goal comes down to controllable traction and elimination of wheel spin, I dont think you need to look outside the window of just adding individual brakes to the existing Case rear axle. Its plenty tough to handle this and has been successfully done by one of the group members (rascal beat me to it!). Someday my 449 project tractor will have dual brakes.... Very similarly done to the fellow members method.
I could be missing your goals completely here.... you really could be planning two rear drive wheels and swivel casters up front, but it did not sound like that. For more overall "traction", I rather like plumbing in disconnects on the existing drive circuits so you could parallel in a 3 point mounted "second rear axle". That will do wonders for traction, but might affect your hopes for great steering !