There are a number of questions here and I'll start for the non-root/stock ROM:
Q1: Can app A use the camera at the same time as app B?
Caution: If your application does not properly release the camera, all subsequent attempts to access the camera, including those by your own application, will fail and may cause your or other applications to be shut down.
Q2: What about apps that use the front and back camera at the same time.
A2: Older devices were hardware/software limited regarding being able to use both at the same time. Device variation made supporting camera difficult. Developers had to check if both were able to be used and the original Camera API was being replaced by the newer Camera2 API. While API improvements have been made, Cameras have always had manufacturer/device specific "cool features" only enabled for manufacturer camera apps and unavailable to third-party app developers both then and now. Android 5.0 was when the newer Camera2 APIs were introduced but that didn't mean all features from the older Camera APIs were supported.
Q3: What about mixing audio from two different apps?
A3: The general concept is called "audio ducking" or "audio focus" and is allowed on Android. I don't know if the app you are referring to is doing more perhaps with legacy Samsung SDK APIs.
Q4: Okay does anything change with root?
Root just gives superuser access to all apps and allows for a level of customization. Unless the ROM is modified the Android framework APIs are still the same. You can see that the low level Camera framework is quite involved and I doubt just having root will allow for multiple apps using the camera at the same time. Realize that Custom ROM builders have to spend time probing the stock ROM to enable standard framework features for upgrading Android OS versions. Any access to "hidden APIs" would be bonus.