Here is a paper from Ansoft on modeling twinax cable, but it does not
specifically address your inter-conductor capacitance question.
A good source for searching for an answer is to look up
. It uses twinax for the interface, and there is a plethora of articles, app notes,
and datasheets available. You might try calling Belden or any manufacturer and ask to speak to an
Applications Engineer. Almost certainly you will get an answer.