Two Voice Trackers can be combined using a simple 3.5mm stereo Y. For better coverage in large rooms, several Voice Tracker I and Voice Tracker III can be combined with a mixer.
Mixers that accept unbalanced, mic level stereo input through a 3.5mm connection is preferred for simplicity, even if simple adapters are required. If the mixer requires an XLR connection, you can adapt from 3.5mm to XLR using a product like this. These adapters will also help prevent ground loops.
Nova Southeastern University used the Extron MCV 121 Plus in dozens of rooms. See: https://www.acousticmagic.com/about-us/user-comments/#alinzels
The Extron MVC 121 Plus is a compact, three input stereo audio mixer featuring a digital signal processing platform for audio signal mixing and control. The MVC 121 Plus features a stereo line level input and two mic/line level inputs, plus fixed and variable stereo line level outputs. It offers gain, filter, tone processing, and parametric EQ. Quick and intuitive configuration using the DSP Configurator™ Software allows the MVC 121 Plus to be installed in very little time. The MVC 121 Plus is ideal for presentation applications that require line and microphone audio mixing with DSP in a small form factor.
Other Integrators recommend the Yamaha MG06X Mixer
Less expensive approaches include simply using a 3.5mm Y. Several universities have had success with that approach.
A more expensive approach is to use a DSP device like the BiAmp Tesira.. See: https://www.acousticmagic.com/news/combining-two-voice-tracker-greater-room-coverage/
This approach is best implemented by a profession AV integrator. Care must be taken in the connections. If you are trying to connect to a DSP, connect the tip and the ring to Positive and Negative on the DSP.
Do not connect the ground to the DSP.