That's fine, but the question you pose involves an equation with substantial emotional value - we have no idea how much value you can place on that dual screen.
Personally, I say absolutely not. If the same price, I would say either the MBP is undervalued or the iMac is overvalued. Definitely take the 2012 as an investment relative to the iMac.
For a dual screen, there are much cheaper alternatives, whether it is a few adapters and a general DVI monitor, or finding a Cinema or TB display (even new). Unless you have some use of dual systems running rendering in the background while operating the other system, you could buy a new TB display.
If you do have that much going on that you could use both systems, buy more memory for your iMac and run things in the background there.