netmeg - 1:14 am on Apr 23, 2013 (gmt 0)
Well you have to think in terms of percentages (because Google probably does) How much of the overall content (including navigation, images, text, products, footers and headers - EVERYTHING) is duplicated from one site to the other? If a majority of the overall content is unique, you're probably okay. If only a small part of the content (a few products) is unique, then you're probably not.
I have six WordPress sites (one each in six states) that have essentially the same templates, styles, type of navigation, plugins and policy pages, but each site has 500+ unique events, and that means that probably 80% of each of my sites are unique, and maybe 20% is the same. And so far (fingers crossed) that has worked for me.
But there's no set percentage that I can tell you that guaranteed works. Search engines don't work like that. I can only tell you my situation.
Start with a small amount of duplicated content (very small) and test it till you find your threshold of pain.