Think outside the box, and use multiple instances of Renoise?
Beat-mixing is actually quite easy when you can control the BPM for both songs.
Hint: search for Hitori Tori videos to see just how much you can screw with Renoise, using a vanilla setup.
If you want to control each instance separately with MIDI gear, you’ll need to launch Renoise with different configurations.
I’ve done a quick guide on how to set this up on a PC running windows:
https://forum.renoise.com/t/multiple-renoise-instance-with-different-preferences-setup/44153