How to make a line perpendicular to two curves

Hello.
I need to set the command rhino "perpendicular to two curves". No how, but I tested doing protgramacion you can see in the picture attached.
The idea is to cade vectors perpendicular curve, but not how to locate vectors having the same direction but of opposite sign, which indicate the ends of the straight perpendivulares the two curves.
It is probably easier but not how.