Dual SIM APN config AttachAPN key

I have an AT&T esim with a Sprint sim, and somehow I just got this to work. The AT&T esim needs a cellular profile that sets the apn name "broadband", and Sprint sim just works, and I tried specifyling its parameters in the profile, but hoped I could just keep whatever Sprint configuration is hidden from me, and just add the AT&T's APN.
The Apple's developer document is really really sparse and I wish they explained, at least what is the difference between APNs and AttachAPN. Now my theory was that since AttachAPN specifies a single entry, and APNs specify an array, the AttachAPN will be for the single sim, and APNs will be for the (possibly multiple) esims. After coming across your post I am reminded that there are iphones with 2 regular sims, so I am no longer sure. Another piece of the puzzle (again, my reading of the apple document), is that if you specify PayloadIdentifier=com.apple.cellular, this will replace the configuration, but if you specify something like com.apple.cellular.dxqz, this will append to the existing configuration. With these two things in mind, I was able to write the script below, which worked for me:

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">

<plist version="1.0">

<dict>

<key>PayloadContent</key>

<array>

<dict>

<key>APNs</key>

<array>

<dict>

<key>Name</key>

<string>broadband</string>

</dict>

</array>

<key>PayloadIdentifier</key>

<string>com.apple.cellular.dxqz</string>

<key>PayloadType</key>

<string>com.apple.cellular</string>

<key>PayloadUUID</key>

<string>ABBCD183-92CD-42BA-91B0-614974B7B445</string>

<key>PayloadVersion</key>

<integer>1</integer>

</dict>

</array>

<key>PayloadIdentifier</key>

<string>my_apn</string>

<key>PayloadType</key>

<string>Configuration</string>

<key>PayloadUUID</key>

<string>30086F47-CB6E-45A1-B365-AA304195C8AE</string>

<key>PayloadVersion</key>

<integer>1</integer>

</dict>

</plist>

However, because of the following I am no longer certain that my undertanding of the script is correct, or why it works. If I add the AuthenticationType=CHAP to the APNs broadband configuration above, it breaks not only the AT&T connection, but also Sprint's. I would love to understand this a little better, and have a script that not only works, but also is correct for what I need it to do. I pass the torch back to you sir.

D.