Nope - never happen.
Reason? The demand to spend 5-6 days at sea, then spend 5-7 days cruising around, and spend another 5-6 days at sea coming back is basically non-existent.
Cruising is all about supply and demand. No one will provide something that does not have a big demand.
What you're talking about has a VERY limited demand, so it's not something the cruise lines find profitable. And, after all, it's all about making money.
Best you can do is to find a transatlantic repositioning cruise that will provide part of what you're looking for and then fly the other way or look for a one-way transatlantic crossing such as Cunard provides.