Follow the steps below:
- Insert the multi-out version of the TR-808/909 plugin (not the stereo out version) on a track.
- On the TR-808/909 control panel click the "options" button and select "sub output" from the popup menu (enable as many of the sub outputs as you want).
- Open the mixer view in Logic - at the bottom of the fader for the TR-808/909 there will be a "+/-" button to add/remove aux channels for that instrument.
- Add as many as you need to match the number of sub outputs you enabled in the instrument.