Hello,
Are you saying that you have friendly urls disabled and you cannot create two blog posts with the same title in two different blog instances? If yes, I don't see how a problem exists because the actual url of a blog post is like: /Blog/ViewPost.aspx?pageid=3366&mid=5785&ItemID=246 so 1) the title doesn't matter and 2) the pageid, mid and ItemID query parameters will never be the same for two different blog posts in two different blog instances.
If you don't have friendly urls disabled, you could manually add something to the blog posts friendly url to specify which blog it belongs to. For instance, if you have a blog for Sales and a blog for Research, and both blogs will have posts with the title "1st Quarter Report," you could have the friendly url for the Sales post be "~/sales-1st-quarter-report" and the friendly url for the Research post be "~/research-1st-quarter-report."
Hope this helps,
Joe D.