I'm willing to bet money that just like in every single past Pokemon generation, both will be catchable.
One will be catchable at lvl 40, the other at 70. That's how they've always done it, and I really don't see, or think they see, a reason to change it.
But, if I'm wrong, you have my most sincere apologies.
More on-topic, sorry about your loss, WPM, and I'd like to say that if Zekrom does end up being electric type, and that cave being his "domain", it'd mean that Reshiram being a whatever (fire? it has red accents...) would have it's entirely own domain, just like Ho-Oh and Lugia in Gold/Silver/HG/SS.
BUT FIRE CAVES ARE SO BORING WE'VE HAD ONE BEFORE T_T'