This is what I could gather from answers on Stack Overflow. I have no development background, so corrections are welcome.
Camera could have only have one object as mentioned here. But this was depreciated from Android 5.0. So, devices running KitKat and below, cannot have this feature.
It seemed to be possible on some devices to open both front and rear camera as mentioned here. However, it is not possible on all devices due to hardware & Software restrictions (see comments)
The Android camera APIs generally allow multiple cameras to be used at the same time, but most devices do not have enough hardware resources to support that in practice - for example, there's often only one camera image processor shared by both cameras.
- The link in your question is the best I could find for a clear explanation.Nokia, Samsung, LG seem to have started this trend on some of their devices. Such devices either need Dual Camera Mode or Dual Sight Mode. There wasn't much I could find about these OEM specific technologies. The Frontback Camera app mentioned in the link, works on Pixel 4a Android 11. Note that the pictures were very grainy and not worth the result IMO.
OP tried apps mentioned here and they didn't work.
This leads me to conclude that it is both a hardware & Software feature and apps can be poor substitute. I doubt if trying on other devices would show better results. Neither have I come across any rooted or Xposed modules that can do this, so it is best to buy the devices that support this natively, if you want this feature badly.