With the spline you will only get a forward vector (direction along the spline), so you need another direction vector that’s perpendicular to build upon. In this case I took the forward vector of blueprint that holds the spline as a right vector, the splines direction at a given distance as forward and up is the crossproduct of both.
Since you are doing this with a circle, something like this might work:
This will only work in this case since this spline is essentialy 2D. It won’t flip unless X and Y cross:
Hierarchy for reference:
This, distributing volumes along a spline and building a BP that extends two splines in parallel are the only solutions I can think of right now. So ideas are more than welcome.
Anyways… Hope it helps.