I have requested this feature often before (during the 4-5 years I have used EA), and I will be happy to add my vote here too.
After the feature of inserting a diagram frame in another diagram was added, I believe that it was possible to EDIT the diagram inside the frame. I thought that was absolutely brilliant. But now it's not possible, and I'm starting to believe it was just something I dreamed.
But judging from the unwillingness that Sparx shows to add the "view multiple diagrams" feature, there must be some basic weakness in the application architecture that prevents it. They must have registered heaps of requests from users by now, unless they haven't been paying attention. And usually they pay a LOT of attention to the problems of the users. So I guess it must be really difficult for them to implement, or they would have done it.
After all, looking at other applications, it's a feature you would assume was there. What word processor, drawing program, etc. would not allow you to view two documents at once? And copy & paste between them?
Anyway, please add the feature now

Best regards,
Mikkel
P.S. @KP: I have tried working with two open EA instances. I believe the reload button was added after a discussion I had with Sparx about this method of working. But like Midnight said, the risk of messing something up and losing work is just too great.