Is there a specific reason you are considering running 2 CMS on your website?
If the website's main purpose is to be a shop, in most situations I would say you should just use /blog/
Most shops don't need such a huge blog with 100 perfectly organized categories, etc.
It's usually just used for inbound content marketing purposes.
Unless you have a specific reason for wanting to use 2 different CMS, then I would probably advise against it in most situations.
But just for the sake of the argument, if it was a blog with an attached shop (sort of the opposite of what you are doing), then you may notice that the attached shop is lacking organizational structure because in that situation, it is usually just one page. It may not convert very well because it may be a bit hard to use, particularly on mobile phones and tablets. In this situation, I could see somebody maybe wanting to have a shop(dot)domain(dot)com
Just my opinion.
