I started off copying the files from Windows, using drag/drop, but this proved to be a little slow (I assume that it was copying the files from NAS1->PC->NAS2). So I found this guide which showed me how to SSH into NAS2, and then use smbclient to pull the files direct from NAS1.
Which seems to be getting about 10MB/s. Which is 80MBit, and so probably about as fast as the switch can usefully manage.
Now I just have to leave it for 12 hours while all 400Gig of videos copies over...
Original post on Dreamwidth - there are comments there.