Not too similar at all to my nose. Some people find there is a note or two that they share, but its not enough to say they share an accord. Really, they are two very different scents. I've owned By before and I own Kenzo Jungle Homme now and I would describe them as such:
By - dirty lavender fragrances; think lavender fougere with some sweet & powdery spices and aromatics with a touch of leather; truly unique
Jungle - distinct oriental that really captures the smells of the East & Orient; starts with spiked lime followed by strong spice notes of nutmeg, cardamom & pepper, finishing with a powdery wood notes and sweetness.