What are you using for "D"?
F = 4DT/L where T is tension, D is deflection, and L is cable length.
So for 1" deflection at 1000# cable tension, you would need 32# force to get 1" deflection. 15# would be 469# tension.
If you're using a different value for deflection, of course the numbers are different. 15# with a 1/2" deflection means 938# tension.
That's not quite correct. Despite not being crossed, the vertical component of tension from the upper and lower wires still oppose each other. Monoplane or biplane, the horizontal component of tension pulls the wings tighter to the fuselage.