Using MixCast Capture without a 3rd Controller
Unlike titles implementing the MixCast SDK, SteamVR requires a third controller to enable quadrant mode (mixed-reality view showing the foreground, background and alpha separately on-screen). As MixCast Capture uses existing SteamVR mixed reality features you will need to use a third controller or the below workaround to use Capture for mixed reality recording.
Thankfully, Second Reality has created controller emulator. This software creates an emulated (virtual) controller that tricks SteamVR into thinking it's a real-world controller. As long as SteamVR shows a third controller in your SteamVR Status panel games should launch correctly with MixCast Capture!
Download the OpenVR-InputEmulator here:
Instructions on setting up the emulated controller can be found here:
If you have a Vive Tracker you can use it instead of the emulator with this workaround.