I think C makes a great second language. Use Python to teach if statements, variables, control structures etc - then later use C to teach low-level algorithms, pointers and how computers actually work.

I don't think there's any harm in learning Python first. I think they may be harm in learning C first - it's much more likely to frustrate people and potentially even put them off programming.

This is the same reason I dislike Java as a teaching language: having to tell people "don't worry about what public static void main(String[] args) means just yet" when teaching "hello world" isn't a great introduction to programming.