Here is a paper from Ansoft on modeling twinax cable, but it does not specifically address your inter-conductor capacitance question.http://ecadigitallibrary.com/pdf/58thECTC/s37p13p5.pdf
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.