AFAIK, Jellyfin cannot combine tracks from two separate streams on the fly. If you can write the streams to disk you can use ffmpeg to combine them locally.
Jellyfin 10.10.3 (Docker)
Ubuntu 24.04 LTS
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
3x WD Red Pro 6TB CMR in RAIDZ1 (JF Library)