Socializing adult cats

We adopted 2 adult cats, a 3 year old and a 6 year old, just over a year ago. They had been with a rescue group for a few years and were bonded. The younger one can be petted in certain circumstances, but doesn’t come to us, we have to go to him. He frightens easily. The older one appears to have been a feral (clipped ear) and cannot be touched at all, runs away when even approached. At first they spent time with us just hanging out in the same room, but lately they have stayed secluded upstairs and we rarely see them. They will each lick treats out of our hands but that is the extent of our interaction, I am looking for advice on how to socialize them, at least to the point they will stay around us.