I don't have Designer, but what I would do is put each Spline in Edit mode so that I can see the Control Points. After exiting Edit mode, I would then put Points (using the Vertex Snap function) at each of the Control Points.
From there, I would draw a new Spline hitting all the Points and be sure to Close to end drawing.
TC Pro Platinum 2020, 2019, 2018, 2017, 2016 & 2015 (all with LightWorks & RedSDK) & V21
System: i7-5820K @ 3.30GHz, ASRock X99 Extreme4, 16GB DDR4-2133 RAM, Gigabyte GTX 970, Samsung NVMe SSD 950 (256GB), Windows 7 Pro (64-bit) SP1