I think if you want to say "ties", you would need to specify what are the ties. Because you say that and the first thing that comes to mind is "What ties". If you say "The ties between a and b are unbreakable" would be better, but only if you already spoke about what those ties before in your text. "The financial ties between a and b are unbreakable".

Ties between a and b are unbreakable.

I am not sure what you are trying to say, but you can say something along those lines like "The bond between Jan and Jim is unbreakable". I am not sure if that's what you are trying to say..?

Both are wrong.