from what i remember, crossposting within livejournal was always manual - make your post, copy the code/text and paste it into a new post for a different comm, add a little note about "crossposted from/to x" if you want, rinse and repeat. looks like it's still the same here. on the one hand that can be nice because it makes it easier to tailor a post to a specific comm even if you're copypasting, but it does slow things down compared to tumblr or whatever. ye olde manual retweets....