Sonix handles multiple speakers depending on whether you upload one track or multiple tracks.

Uploading one track with multiple speakers
​Using a single track recording with multiple speakers, Sonix will break up the transcript into exchanges. You will have to manually enter the speaker name and manual label each exchange, however you can auto-fill one speaker entirely. 

For more info on speaker auto-fill check out this article: https://help.sonix.ai/editing-a-transcript/how-do-i-auto-fill-or-auto-label-speaker-names

Uploading multiple tracks
For each individual track you upload, you can name the speakers. Sonix will combine each track into one transcript and automatically label the speakers throughout the entire transcript. Using multiple tracks results in more accurate transcriptions.

You can enable multitrack here: https://my.sonix.ai/account/profile/multitrack