Pierre,

The matched loss of the resulting open-wire line is not related to the
matched loss of the coax cablebecause those cables are not
individually operating in "Transmission Line" mode. For the same
reason, the velocity factor of the open-wire line will not be the same
as the coax.

Here's a write-up of an experiment I did to produce low Zo balanced
line. I didn't measure the loss because my application was for a very
short run:

http://www.karinya.net/g3txq/twin_feed/

73,
Steve G3TXQ