Blending 2 lines

Hello, I have a problem when blending two lines. I have 2 lists with the lines I want to blend, but the issue is, one list is twice as big as the other.

The first list has 24 curves, and the other one 48. I want to join both curves.

This is the 3d: https://gyazo.com/6317bf58cbfe7d418ebfcc06f7ae7e3e

This is what I want to achieve (black lines): https://gyazo.com/52a652dd9074ca5ac62dba6caf526edc

Basically, its the Beijing Olympis Stadium's structure.

Sorry if it's not well explained, tell me if you don't understand anything.

Thanks in advance!