The main character gains psychic abilities when he enters a cave on an alien world and a parasite merges with him. Subsequent eeg scans show he has two traces, one normal, his own, and a second which has all the psychic abilities, his parasite. It's not much to go on, other details are fuzzy and may only confuse the issue if they are from a different book.
This sounds like it might be 'Healer' by F Paul Wilson.
The second mind in his head comes from a cave creature that drops from the roof of the cave and joins with whichever type of animal that has wandered in. The symbiote gains the same intelligence as the host animal.
There is also a quotation that is floating about in my memory - 'Of every 100 people that are attacked by the cave creature 99 of them will die'. Then it goes on to say that the 100th will not die. The symbiote can effectively make its host immortal....
Sound like Dan Simmons' "Hyperion", Part One, The Priest's Tale: "The Man who Cried God".
I'm still not sure we can call what Father Duré got a psychic ability, but at least some action happens in the cave.