OFFICIAL PAGE: DeMIX Pro
DeMIX Pro, from AudioSourceRE, is an excellent application for upmixing mono source material to stereo. DeMIX Pro combines cutting-edge sound source separation capabilities with a built-in advanced spectral editor. Version 4.0 of DeMIX Pro now adds separations of strings and piano, improved drum and bass separations, as well as the ability to perform your separations locally on your computer, or in the cloud. DeMIX Pro is unique in that it can separate the lead vocal from not only the backing instrumentation, but also from the backing and harmony vocals! A multi-channel mixer is included which allows you to quickly and easily listen to your separations in the context of a stereo mix. Level, pan, mute, and solo controls are available for each separated track. DeMIX Pro is available for both macOS and Windows. Full details at the link above. Highly recommended!
Screenshots courtesy of AudioSourceRE