I took a brief look at the current state of such connections for our Coalescing Minds paper:

3.1. Direct brain-to-brain connections. The easiest approach seems to be to connect human brains directly in much the same way as the two brain hemispheres are connected. The corpus callosum, which connects the hemispheres comprises 200–250 million axons crossing from one hemisphere to the other. It seems likely that to coalesce minds, the number of connections should be of a similar order of magnitude, probably at least millions.

The technology exists today for creating hundreds of connections: e.g. Hochberg et al. [2006] used a 96-microelectrode array which allowed a human to control devices and a robotic hand by thought alone. Cochlear implants generally stimulate the auditory nerve with 16-22 electrodes, and allow the many recipients to understand speech in every day environments without needing visual cues [Peterson et al. 2010]. Various visual neuroprostheses are currently under development. Optic nerve stimulation has allowed subjects to recognize simple patterns and localize and discriminate objects. Retinal implants provide better results, but rely on existing residual cells in the retina. [Ong & da Cruz, 2011] Some cortical prostheses have also been recently implanted in subjects. [Normann et al. 2009] We are still likely to be below the threshold of coalescing minds by several orders of magnitude. Nevertheless, the question is merely one of scaling up and improving current techniques.