You must set the parts to use the same midi-channel if you want to layer the sounds. If not you will only hear the part (or midi-channel) you selected.
About the sound sounding bad; Have you checked that it has the same reverb/delay-settings in single- and multi-mode? In multi-mode you only have one choice of reverb/delay-setting and one send pr. part. You may find that one sound uses reverb in single mode but then uses delay instead because the multi uses delay. You can use the funtion "copy reverb/delay from single" witch will (as it says) copy the reberb/delay-parameters from the single. Of course you may then find that the other sound dosn?t sound as intended instead
In the new TI?s this won?t be a problem since it has seperate reverb/delay-settings pr. part...