I agree with the previous suggestions to use either a numbered or long switch for smoother transitions from one track to the next.
However, if you're intent on sticking with FasTrack, the narrowest you can make the two parallel tracks would be to use an O31 switch and 3x O31 Quarter curve pieces. The total width of this arrangement would be 8 3/4" wide from the outside edges of the roadbeds like so:
If you were to try to use an O36 Switch with O31 Quarters, the narrowest you could get would be about 9 1/8" OD.