How to Find Tangent from Multiple Curves

Hi!

As can be seen in the screenshot, I have these multiple curves forming a curve surface.

However, I have 2 different curve surface (on the right and left side). Is there any way to connect these curves, such that it can mantain its continuous surface?

I was trying to do this by using the bezeir span. However, I didn't know how to find the tangent for curves. Any idea how to do that?

In addition, as one of the input for bezeir span is the starting point and ending point. How to make sure that each of these points that will be connected, share the same y-values?

Thank you so much!