Is it possible to use the same username in configuration of different lines of different sip providers?

No its not possible with Express Talk, but possible with third party softphone like eyeBeam..
When trying to setup your lines in Express Talk you will get the following error: *see example:
"Invalid Data Entered"
"Express Talk does not support registering 2 lines with the same user name, but you have set up an extension '1024' for more than one line. Default Line Settings is registered with '192.168.1.28', while Line 1 is registered with '192.168.1.253'. Please remove or modify one of these lines to continue."

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