D10 Controller described confused similar callsign event when an aircraft accepted a descent clearance issued to another; both ATC and the involved flight crews missed the mistake.

Narrative

Conducting OJT on both FW1/2 and HFW1. ACR X was level at 11000 15 nm NW of SILER. FW Controller descended ACR Y at SILER to 6000 and switched to 118.42(AR3). Apparently ACR X answered and began descent. The HFW developmental was highlighting ACR X because he observed mode C at 10900 then 10800. ACR X reported on AR3 frequency and AR3 began inquiring about his correct frequency and location. AR3 climbed ACR X back to 11000 and switched back to FW. Contributing factors to this deviation were: 1) Both developmentals were competent; nearing the end of training. We were 'getting time' to meet minimum requirements for HFW. These circumstances conspired to make me inattentive. Instead of behaving as the 4th level of redundancy; I should have been totally focused on the operation; especially since it was an easily discernible potential for confusion in call-signs. 2) Pilot inattention. Pilots have to share responsibility and maintain acute awareness. The pilot should have been aware that he had been following the similar sounding call sign for 300 miles and he should have been aware that he was 15 miles from the normal descent point which might have made him simply listen for the other aircraft to acknowledge. 3) Pilots should be encouraged to question when potential confusion exists. ACR Y probably heard the call and knew he was at the correct descent point.