Cross Posting Between main/subdomains

I am trying figure out the best way to manage a main site and a subdomain for that site. After some reading on the forum I am guessing what I am asking won’t work, but as I am not terribly technical, some of the discussions went over my head and perhaps I was misunderstanding – and what I want IS possible.

So I have a network with a main site at kinkandpoly.com. I have it set up so that recent posts from three specific categories pull into the Home Page in separate areas, and this seems to work fine. One of the “categories” is actually a category that I have created a subdomain for, kinkandpoly.com/jademelisande. I want this to be a separate site because I need to have an entirely different theme/look and pages for it. I have created it, and it seems to be working so far (except some glitches in the theme itself, which I am working out I hope.)

My questions:

1. I would like to port over the posts that have that category into the subsite (so that appear in both blogs.) Is there an easy way to do this (without exporting ALL the posts from the main site)?

2. Is there a way to have the posts I write in the subdomain show up in the main site, in its category, automatically, or will I have to copy them over by hand?

Thank you.