(Once again the "Questions that may already have your answer" worked and I found this question before asking it again.)
It's been two years, and we now have more user pages: I'd like this feature even if it only worked for our own user pages.
In other words, this OP's suggestion of all pages where possible link to corresponding pages in the other site would be great, but even if it is just that, when you're displaying any of your own user pages, the meta/main link actually just goes to your corresponding page on the other site, it would be good.
Even if it just went to your user summary it would save one click. I understand and agree a small a change as that would be bad for UI simplicity and understanding, but I'd prefer it...
Of course, I'd prefer more, though, as I said above, it doesn't have to be the whole site like the OP suggests.