Could it be done serially? Singer 1 annotates, layer 1, sync; singer 2 annotates, layer 2. sync, just switch tablets between syncs. That's probably what Mike is suggesting.
Dell Latitude 13.5" 2-in-1 Ubuntu/Win 11
Samsung Note Pro SM-P900 12.2 Android 5.0.2
Samsung S7+, Android 12
Samsung Note Pro SM-P900 12.2 Android 5.0.2
Samsung S7+, Android 12