ZHU Controller described an inappropriate use of visual separation during a climb through occupied altitude when one aircraft was NORDO.

Narrative

I was working the Cll Sector which is 8;000 and below. A BE35 was northbound IDU-CLL at 4;000 FT. This aircraft was NORDO. I had another aircraft go to guard frequency (121.5) and try to raise the aircraft. This was unsuccessful. Aircraft X departed Runway 11R southwest bound assigned 3;000 FT. I RADAR identified Aircraft X and issued traffic at 4;000. I also asked him if he was in the clear. He said he was in the clear and subsequently reported the BE35 in sight. I asked if he could maintain visual separation and he said he could. I told Aircraft X to maintain visual separation from the Bonanza and climbed him to FL230. Aircraft X acknowledged this clearance and started the climb. When Aircraft X was through about 3;500 and passed Aircraft Y I suddenly realized my error. I did not have to be talking to Aircraft Y but I had to have the ABILITY to talk to him. Since he was NORDO I didn't have that ability and therefore according to the rule could not use visual separation. Recommendation; none. I have used visual many times when only talking to one of the aircraft. NORDOS are rare. I've never encountered a situation where these elements came together before.