Curious question

I'm not certain, but you might want to set this up as a collaboration or shared-world exercise from the start. With legal paperwork outlining who owns what. I suspect that agents and editors might be a little nervous about this, because they won't know the quality of your friend's writing, and your friend basically owns the world along with you. One issue would your friend writing and submitting something to a different publisher.

My own past experience with almost this same situation led to that person no longer being my friend, and me having to not only scrap my first novel but completely re-invent my world to avoid legal issues. What hurts is that it was my world to start with, and I let her 'play' within it.

I would say nothing. Make a deal with your friend that the first to publish will do what he can to help the other get published, but either is free to write in the world as they see fit and you both have full rights to the material to use as you see fit. Write it down, just to be on the safe side.

But don't bother mentioning it to an agent. As long as you have rights to use the material they probably don't care. You can always fill them in after signing with them if you want to.

This sounds like a situation ripe for legal complications. In the case of a series, you're looking for one publisher, which means said publisher must approve of both your books. Could be a problem if your skills aren't on the same level, or even if your styles are different.

For your two books to be part of a true series, they'll have to share more than common zombie tropes (zombies are created by viruses/radiation/voodoo! zombies can only be killed by destroying their brains! and the like.) Shared characters, shared backstory, shared agencies, are what I'm talking about. And if you're sharing these, hence a true interdependent series, yeah. You need to get some terms down in writing ASAP.

You'll also need a series bible to ensure consistency. For example, you can't destroy your NYC zombies with chamomile tea, while his LA zombies drink chamomile tea three times a day, with relish.

Lots to think about.