If you're using the Xcode 12.5 beta, you should be able to add the "SCSIControllerDriverKit" framework to your dext, and then include the necessary files in your code using "#include SCSCIControllerDriverKit/..." Do you not see these options in Xcode 12.5 beta 3?
FranticRain
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IOUserServerOneProcess only coalesces dexts with the same provider, so you'll only see it if all of these dexts have the same provider.
If all of these dexts are under a singular USB device, then you can match to the device and manually match to all of the interfaces in your Start implementation. By creating your interface instances manually instead of matching via plist, then it will all be running in a single process and it should be easy to transfer data around.
Alternatively, or if they aren't all under the same device, you should be able to use an IOUserClient to communicate between the different dexts. See https://developer.apple.com/documentation/driverkit/iouserclient.