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 MlL-STD-1553B
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.