Looking at the Owner's Manual and Reference Manual, I assume that by AUX OUTs you really mean the two jacks that say "AUX OUT" in parentheses, or LINE OUT SUB 4 and LINE OUT SUB 3. If you set those two jacks to function as AUX OUTs-- as shown in the upper right corners of the illustrations on page 124 of the Reference Manual, where "AUX OUT" is highlighted in orange instead of "SUB3-4"-- I believe that those two jacks will just output whatever audio signal is coming in through the AUX IN jacks, sort of like a "MIDI THRU" port except for audio instead of MIDI data.
Based on the information and illustrations on that page, it looks like you should be able to select the "SUB3-4" option for that field in the upper right corner of the MIXING CONSOLE screen, then check both the MAIN and AUX OUT columns of the MIXING CONSOLE grid for each of the keyboard parts that you want to send to both the MAIN output jacks and the AUX OUT (or SUB 3 and SUB 4) jacks. Although I have no first-hand experience with the Tyros models, it looks like you're able to selectively output the various keyboard parts to whichever of the audio jacks you wish, such as sending RIGHT 1 and RIGHT2 to only the MAIN jacks but sending RIGHT3 and LEFT to only the AUX OUT jacks, or something like that.
EDIT: I kept saying "AUX OUT" because that's what's shown in the two illustrations, but for all I know the last column of the MIXING CONSOLE screen may change to say "SUB 3" and "SUB 4" if you've selected the "SUB3-4" option.
