As others have said, yes, you can run two parallel lines. An easy way
to think about the impedance: assume the lines are independent, and
each has a matched load equal to Zo. Parallel them, and the load is
Zo/2. Since a (uniform) line with matched load presents a Zo load to
the source, the source with two such parallel lines will see a Zo/2
load.

BUT...BEWARE! What if the two lines are not equal electrical lengths?
The results may surprise you. Details are left as an exercise for the
reader. You can use standard transmission line equations, or use a
program like RFSim99, to see what happens. Try, for example, a couple
of lengths of lossless 100 ohm line with v.f. = .66, one of length =
100 feet and one of length = 101 feet. (I'm curious what others get
for results...)

Cheers,
Tom