If a novel is cross-genre, one of the genres must be the strongest and its genre tropes and plot must move the novel forward throughout. This driving plot is the plot the main character follows to achieve his/her goal.
A werewolf novel that is driven forward by the worldbuilding and various werewolf political/pack struggles is urban fantasy or horror. A werewolf novel where boy wolf meets girl vampire, and they fall in love during various werewolf and vampire struggles is a paranormal romance.
You must understand what the central genre of your novel is so your novel doesn't fail by genre standards, and you will know where to market it.
You also have to understand and have read both genres you mix. Cross-genre is not only supposed to mix the two genre, they are supposed to mix the two audiences. Insult half that audience by not knowing your stuff, and there goes sales.