Given that BeReal was getting so popular, it was probably only a matter of time before the incumbent players in this space realized that BeReal's claim to fame is actually more of a function than it is a standalone app or network, particularly because it isn't algorithm-driven.
And functions can be copied, and distributed to many more users than are able to participate in the standalone apps.
According to Snap:

"With Dual Camera, Snapchatters can create content using their front and back-facing cameras simultaneously."

Which is, of course, pretty much exactly like BeReal, and Instagram's new Dual/Candid options, though Snap is looking to push the functionality beyond situational selfies, and more towards participatory, creative trends.

That is to say, in substance, that Snap is yielding and copying from the rest, but it is at least attempting to give its take on its version, unlike merely appropriating the same function for its app.

Still, it is also the same thing.

There will be several Dual output formats available to users including vertical, horizontal, picture-in-picture, cut-out.

And besides the improved creativity powers of ordinary users, this can also make available much more interesting possibilities for UGC, allowing brands to invite people to participation in all-new ways that can contribute to new trends on the app.

Snap had previewed the feature actually at its Partner Summit back in April as part of its 'Director Mode', which offers more advanced editing abilities within the app, to enable more creative expression 
The director mode will also, in time, enable 360 content creation, as Snap leans into the next stage of digital creation, with more immersive, all-encompassing camera tools.

Although where exactly Snap positions itself in this regard is not apparent, with the company also compelled to slow hiring and cut projects due to global economic impacts and limitation resulted by Apple's iOS opt-in data privacy update.

The switch has also made Snap ads less effective, and advertisers have therefore cut back on their spend in Snap ads. Though the platform is still unique and has a valuable reach among younger audiences, it's on its back foot in that regard, which could delay some of its more advanced projects, including, potentially, its AR-enabled Spectacles.
This places it in keeping with the hopes that Snap is also looking to breakout some of its Director Mode features into the mainstream app, as Dual mode also relates to wider usage trends, and could be a valuable engagement-focused addition.

Which is also in part why Snap is 'front and center' in the function bar in the app, rather than as an add-on Lens option, to hopefully encourage its use.

Does it become a huge trend on Snap? While better general trends may suggest an interest, and the more unique varieties of presentation do unlock new content possibilities:

Snap has had a pretty good history at knowing their audience and bringing in trends that would fit their behavior, so it could be very valuable indeed for the app.

And as noted earlier, it also opens up many more possibilities for brands, and could be great for complementary ads in time for the holiday shopping spree.

Snap's Dual camera mode is now available to all Snapchatters on iOS with an iPhone XS/XR or later. Availability on Android will follow in the coming months