Two-point discrimination is an interesting thing to play with. In practical classes at uni, we did two-point discrimination using a compass (the drawing kind, not the navigational kind...). Spaces outside your sensitive areas would have the two points perceived as one quite early - in some areas of the back, the two points could be 10cm or so apart. Conversely, on the lips or fingertips, we couldn't get the points of the compass close enough to have them felt as one point.