We've dealt with a bit of overlap before — we've got a whole category on Offbeat Bride dedicated to "mama brides" — but adding Home to the mix made things… complicated. Where do the posts about wacky nurseries and kids' rooms go? (Usually Offbeat Mama, since we figure all Empire mamas read Offbeat Mama, while not all Home readers care about nurseries.) Where should we put that awesome DIY centerpiece post? (On Offbeat Bride, but with a link from Offbeat Home.) When we write about general relationship/communication issues, do they go on Bride (where most everyone's in a relationship) or Home (where the range of reader relationships is wider, including unmarried, engaged, and married — but also singles)?
The editors and I have a weekly editorial video conference where we talk about all three sites' upcoming posts, and try to identify the cross-over appeal posts. If a post has a lot of visual impact, we'll do a same-day cross-link — for instance, when we featured this awesome camping-themed kids' room on Offbeat Mama, we did a quick post on Offbeat Home linking it.
If a post has potential relevance (e.g., a flower pressing project on Mama might be of interest to Brides, but isn't like OMG SO RELATED), we'll include a link in the Clicky Links section of a Monday Montage post.
I'm leery, though, of cross-posting too much. There's a decent number of readers who trawl all three sites (Note to self: this winter's reader survey will tell us how many!), and the last thing I want to do is reward these dedicated readers with irritating reminder posts to read things they're already aware of.
In summary, while I wish I could say there were hard 'n' fast rules about overlap content gets posted, sometimes it just comes down to "Who needs it more?" Offbeat Home, as the newest site, has the lowest traffic and the smallest number of submissions coming in. I find myself often leaning toward posting crossover posts there, as a way to drive eye-balls and content to the site as it's still in its first year of growth.
I wish I could say I had a better defined strategy, but it's definitely something I'm still trying to figure out. Any suggestions?