Looking on http://leagueoflegends.wikia.com/, it seems that Elyse, Nidalee, and Jayce and classified as both melee and ranged champions because they can switch between ranged and melee auto-attack modes. Does this mean that these three champions can buy both Runaan's Hurricane and Ravenous Hydra? Can they have both items at the same time? If so, how do the item effects work when they are not in the right "mode"? (i.e. Nidalee has Ravenous Hydra while she is in ranged auto-attack mode)
The effects only trigger if you have the matching attack style. So Jayce will proc the Hydra passive while in Hammer form, but not the Runaan's. In Cannon form, he procs the Runaan's passive, but not the Hydra. The same applies to each other hybrid attacker with their respective forms.
He will, however, retain all the stat bonuses from both regardless of the form.
Fun fact: there use to be a small glitch where, if you time a transformation perfectly as a ranged attack hit, you could proc the hydra off a ranged attack.
Source: I would play a lot of Jayce on Twisted Treeline and built both to troll a bit if we were crushing.
They simply don't work when you aren't in the right form. (the stat bonuses still apply)
Hydra will only work in Melee form and Runaan's will only work in Ranged form meaning its basically a wasted item slot because the stats without the passive use aren't enough to make up the damage loss