Tiger & Barnaby’s duet will be played during the Hoshizora Sunrise radio show that airs August 21st at 23:00 JST. According to the Twitter update, the song should play around 23:30 JST. You can find instructions how to listen to the radio here.
Tiger & Barnaby’s character song single is coming out on August 24th, more information with samples of the solo songs here.
(I’m looking forward to this! ✧･ﾟ:*o(≧ω≦)o*:･ﾟ✧ Both Morita and Hirata are pretty good singers so I have great hopes for the duet!)