The "Classic" and the "Beta" texture resource packs can both be added to a world to (presumably) run concurrently.

There is no point to doing this - the two packs completely clash (I did it by mistake).

The effect is that the texture for the pack last added (and thus on top of the list) is displayed.  

gives you the classic (old) texture, while

displays the modern updated texture.

I didn't play with either setup for very long as I was concerned that both packs were being applied sequentially and result in a drain on my phone memory sufficient to crash the game.

As mentioned above, there is no reason I can think of for loading both, but it is not difficult to do by mistake.

If the game ignores the first-selected one and only loads the latest, no problem exists(I don't know how Bedrock handles resource packs).