Hi Tommy,

Not exactly. I think I misunderstood your original question.  I thought you had two pages with the same content, and they were accessible via two different categories.

But I think you're saying you have one page, but you can access that one page via the two different categories, but the breadcrumbs are the same no matter which route they took, whether through A or B, they show category A breadcrumbs.

I wouldn't worry so much about the breadcrumbs, I would worry more about duplicate content and urls.

Let's say you're selling a flashlight, and you just have one flashlight product page. But, because of the content of your site, you listed it under two different categories.  Let's just say the categories are tools and gadgets.

So if you had two urls:

http://www.site.com/tools/flashlight and http://www.site.com/gadgets/flashlight

but they were technically the same page (same content and everything just different url), this would be bad.

The fix for this would be to pick the url you want to rank, then put that url as the canonical for the other, so when google crawls it, they know you prefer the other url.

However if it were the same url, no matter which category they came from, there is no problem, because there is no duplication.

Now back to the beginning

If you really want the breadcrumbs to reflect which category they came from, instead of just redirecting to category A, then create another page for category B, make it identical to the page for category A. But on the new page, put the url of page A as the canonical on the new page for B.

So users get the same product page (content speaking) with the breadcrumb that reflects their path, but Google will only count one url no matter which one they crawl.

Breadcrumbs are more for UX like you say, however they do help search engines crawl your site's pages better as well, especially if they're not in main navigation.

I think the canonical issue is the more important one rather than what links appear in the breadcrumb.  I would select which page you would prefer to rank, then put that url in the canonical tag of the other page.

So the canonical would be for Google, and the breadcrumb would be for user.

Also, who knows, maybe having the different breadcrumb is better for the user, because they came from a different path to that product in the first place. But Google would count both pages as the same.

It's my understanding that you treat each subdomain as a unique site. So each subdomain should have its own unique XML sitemap and robots.txt file, as well as submitted separately to Google Webmaster Tools. But to answer your inter-linking question, I would avoid including the other domain URLs in those files (XML and Robots). Only include the URLs for that particular domain and/or subdomain. With that being said however, I would inter-link them on the actual site somewhere. Maybe in the HTML sitemap, navigation, footer, or even naturally throughout your body content where appropriate as well.