Only mono inputs in Logic's (11.2.2) external instrument audio input pop-up

I am trying to integrate two external synths in Logic using the external instrument plugin. Their L and R outputs are connected to the interface's analog inputs 1, 2 and 3, 4. The MIDI part works, but the audio input pop-up list only shows the Fireface's mono channels. After 94 the list ends, no entries like 1-2 are visible. What must I do to make Logic use stereo inputs for the external instrument?