Two Orbi Networks (Two Internet Modems, Two SSIDs) - Backplane conflicts?

I do not know if you have created what you wanted.  Attached is a picture of how Orbi uses WiFi frequency:

Notice that Orbi gets "maximum" throughput on 5G by selecting channels 36-48.  Manually selecting 44 and 48 means the two Orbi's almost completely overlap.  You might be better off to let them go "head to head" with a full 80MHz bandwidth.  Notice also that the only 80MHz backhaul frequency available is channels 149-161, which often goes by "155".

5G band UNII-2-Ext is a "DFS" frequency Band.  Scroll down to 5G on this Wikipedia page: https://en.wikipedia.org

/wiki/List_of_WLAN_channels DFS means that the access point has to constantly scan for weather and military radar and "move" if it is detected.  There have been long discussions about this, and Orbi appears to properly follow the guidelines.

There is a method for setting which channel the Orbi will start with for backhaul.  Maybe if there are no radars in your area it will stay there, as follows:

You can check the starting channel in your Orbi by enabling telnet in Orbi Router and check the following:
> config get wla_2nd_hidden_channel

 You can also change the starting channel, the channel that Orbi will start with after restart as a backhaul channel as follows:
> config set wla_2nd_hidden_channel=108      ==> This will change the backhaul channel to 108 (5530–5550 MHz)
> config commit

Having multiple internet links is a wonderful idea.  Having two SSID's I'm not so certain of.  It might be worth looking toward a Ubiquity Edgerouter which can combine two internet feeds into a single feed.