I try to use a vocoder,but only one be usable between modulator and carrier(Luckily,I can use MIDI).
So how to use sidechains(audios)?
OpenMPT currently doesn't have support for sending audio to anything but the first stereo input of a plugin, so sidechaining is not directly possible. However, some vocoders can still be used because they accept mono inputs for carrier and modulator, in which case the two signals (voice and carrier sound) need to be panned hard-left and hard-right (or vice-versa depending on the plugin) before being sent to the vocoder. One such vocoder that can be used with OpenMPT is TAL-Vocoder (https://tal-software.com/products/tal-vocoder).