I'm afraid the result will not be 100%. If you put the RDS to only one audio channel, internal stereo coder of the BH1415 (that is based on periodic switching between the channels) will cause 50% of the RDS signal going lost. This will result in erratic RDS reception and that's something you surely don't need. There's also a limiter in the BH1415 which is used to cut off signal overshootings (but it does not work too well
). If you put audio and RDS at once, the limiter will cause additional downgrade of the RDS.
Personally I think you should put the RDS to both channels and put audio to both channels. In other words sum RDS and audio (mono) to one point (using for example 1k resistors) and connect both Belkin input channels to this point. Use only low audio level to avoid the limiter action. This will ensure that RDS reception stays error-less and you'll be able to listen some audio, although not loud and superior quality.