It's really really hard to 'aim' a 6km deep hole. Hitting a slab of tough rock can deflect the drillbit off course. If you remember the Chilean miner rescue in 2010, who got rescued by a new hole being drilled - they had 3 seperate holes being drilled at the same time as backups, just because they were worried about the success rate of actually hitting their target. And that was only 0.7km underground.

directional drilling could work if they started nice and high up and aimed to meet somewhere, but turning 90 degrees at the bottom of a 6km borehole is going to require a 6km long directional drilling rig, and probably a fair bit of extra depth to make that turn. it might be possible, but at those depths everything becomes much more difficult to the point where it might make more sense to just give up and abandon the project.

it does like a good opportunity for the fracking techniques mentioned elsewhere in this thread - drop some explosives down those boreholes and see if you can artificially increase the porosity.