This is quite a difficult question to answer as wireless/WiFi environments around a Home can vary so much, but a reasonable proximation to replicate the HT Ad-Hoc 5Ghz ‘bonded’ link, might be to place an available Sonos speaker in the chosen locations and a wireless access point near to the location of the Soundbar and see what SNR reading shows in the Sonos App, over a 5Ghz only connection. You ideally need 45dB, or higher. See this link:

https://support.sonos.com/en-us/article/selecting-a-location-for-your-sonos-speakers

See this support link below aswell, which shows where to locate the SNR reading in the Sonos App.
https://support.sonos.com/en-us/article/understanding-the-network-details-section-in-the-sonos-app

Personally speaking though, I would just try the setup and see if it works, particularly if you opt to buy the 2 x Era 100s direct from Sonos, within the terms of their 45-day money-back guarantee, that’s if the speakers do not work for you at the chosen locations. See this link:

https://www.sonos.com/en-us/legal/terms#legal-terms-sale-warranty

The 5 GHz WiFi has a lot better range than Bluetooth, I’d bet it would work for you.

Are you sure you want them as rear channels?

\If instead you make them a stereo pair and then Group them with your sound bar you’d get the full front channel sound in your kitchen for TV, with a slight 75 ms delay, but they would be in perfect sync for playing music sources. It would also be easier to play one, the other or both set up this way.

Keep in mind that the speed of sound is pokey at about 1 foot per millisecond.

If you have location ‘A’ and ‘B’ separated by 40 feet, the listener at ‘A’ will claim that ‘B’ speaker is 40ms late. likewise the listener at ‘B’ will claim that ‘A’ is 40ms late. Listener ‘C’ at the midpoint will claim no delay. All listeners are correct.