Since the curve is bending and changing the X and Y directions along its path, you cannot simply move in X and Y to keep the second curve offset from the first.

Like a two lane road, where it bends the inner lane has to be shorter and the outer lane longer. You can achieve this by offsetting one curve from the other in the curve's normal direction.

In my example I used a curve and to visualize it I instanced cubes along their path. I made two cube lanes, one offset to one side of the curve, the other to the opposite side:

[![offset curves][1]][1]

To achieve this I scaled (with a positive value for red and negative for blue) the _Normal_ of the curve and used this as _Offset_ for the _Set Position_ nodes for both curves. The cube instances I aligned with the curve tangent.

[![node setup][2]][2]

[<img src="https://blend-exchange.com/embedImage.png?bid=9pg07XnX" />](https://blend-exchange.com/b/9pg07XnX/)



  [1]: https://i.sstatic.net/YCugz.jpg
  [2]: https://i.sstatic.net/mlQzF.jpg