From an RF perspective adding a secondary WLAN system to the site is the worst solution.
Not only do you have interference from 3rd party Wi-Fi systems, that you don't control, but you also add an interference source potentially directly next to the Aruba APs.
I also recommend to look for other solution to address the problem you are trying to solve, e.g. clustering with small gateway to provide redundancy, or install more APs to take advantage of secondary coverage. Depending on the problem you want to solve with the backup system.
If a backup system needs to exist, then try to separate the channels from both systems, so that both system won't use the same channels. This eliminates co-channel interference, at the cost of having fewer channel to be used. Aruba can do it, not sure about the backup system. Also it's dependent on the geography you are in, in terms of available channels.
Every automatic channel assignment mechanism is reactive and connectivity problems will be one trigger to change the channel. the risk of having a poor user experience is high, when sharing channels on two systems.
On top of that both systems will most likely detect the other system as rogue network and throw warnings ... which leads to ignoring those events and a real rogue may go undetected.
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If you have urgent issues, always contact your Aruba partner, distributor, or Aruba TAC Support. Check
https://www.arubanetworks.com/support-services/contact-support/ for how to contact Aruba TAC. Any opinions expressed here are solely my own and not necessarily that of Hewlett Packard Enterprise or Aruba Networks.
In case your problem is solved, please invest the time to post a follow-up with the information on how you solved it. Others can benefit from that.
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