The two tracks form a delta network. The effective differential
impedance is the parallel of (a) the trace-to-trace impedance
and (b) the series of the two traces' impedances to ground.
If the traces are equal and well-separated this is just twice
the individual line impedance, which would mean that your PWB
traces should be in the 55-67 ohm range (call it 61 ohms plus
or minus 10%). If your native impedances are higher, you can
get into the right neighborhood by running the pairs closer
together so that the coupling capacitance lowers the total.
-- D. C. Sessions email@example.com