Available as an assembled Module and as a DIY project.
This Module is currently available.
Crossfader with build in VCA
The X-Fade / Gate module is a very simple utility module that offers Crossfading between two audio or CV sources with build in volume over both inputs, essentially a built in VCA for the master output. A switch can select the two function modes, AC or DC coupled for a wide variety of functions.
These merchants probably sell this module. Huh?