In reply to Matt F:
I spent 4 weeks in east greenland (basecamp was on the shoreline) with BSES last summer. in addition to my boots, I took my old running shoes. Assuming you think the risk of your boots breaking is low, I think this is probably your best bet. Some of the others took sandal type things which were also ok, (off the ice it is likely to be quite dry). If I was going again I wouldn't bother with a second pair of shoes at all. Boots can be worn casually, around camp, by tying them losly below the cleats, and this works well for getting water toileting etc. On the ice/snow you won't dream of anything except boots even for a very quick midnoght venture out of the tent.
I personally found my trainer most usefull for the couple of times I swam out to icebergs, as the rubber sole protects the feet somewhat from the cold and sharp nature of ice!
Enjoy your expedition, it's fantastic out there