Hi gang. Unfortunately elrond, the information stream in the corpus callosum is highly preprocessed and we have no idea what the cortical code in the CC represents, so interfacing to that area will be a long while yet. Even just providing a straight connection electronically would require massively invasive surgery, huge numbers of tiny electrodes, and a lot of upstream and downstream filtering.
If you really wanted to stretch it (pun intended) you could literally grow neurons through mechanical stretching, and surgically attach them to create artificially constructed siamese twins. But once again, massively invasive and number of surgeries required would probably be prohibitive. Sounds a little Gigeresque too - ewwww
Also, this type of interfacing would probably be very likely to cause some form of mental instability if done in an adult brain. Those type of dense interconnectivities probably have to develop over an extended period to be dynamically stable. Suddenly interfacing two extremely complex systems with no transition is probably not going to turn out the way you would hope.
Seriously, the best bet for interfacing in the near future is the sensory systems because the coding is fairly straightforward (as compared to the cortex) and the interface devices are already well underway. Nobody has any idea how to get information directly into the cortex yet, except through the sensory systems - the way it gets in naturally. Midrange (25-50 years) you will probably see the hippocampal interfaces start to come into play. Just shared sensory experience at light speed is going to offer quite a bit of opportunity for group-think if that is what your after.
Best,
Peter