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.