Intermediate questions re: using two feeds to update one content type

Hi all,

So, I have an issue on my hands that I'm sure someone can solve. I have two XML feeds that contain partial information about my products - what I would like to be able to do is merge the data from the feeds into the same content type. Unfortunately, I've been unable to do so.

Feed #1:
GUID
firstName
lastName

Feed #2:
GUID
Height
Weight

...so basically, Feed 1 creates a series of nodes:

Node 1:
GUID: 123
firstName: Fred
lastName: Sanford
height:
weight:

Node 2:
GUID: 124
firstName: Desi
lastName: Arnez
height:
weight:

...but then, when I import Feed #2, here's what happens:

Node 1:
GUID: 123
firstName:
lastName:
height: 5'9"
weight: 180lbs

Node 2:
GUID: 124
firstName:
lastName:
height: 6'0"
weight: 210lbs

I have "Settings for Node Processor" set to "Update existing nodes (slower than replacing them)" - so this is a real head-scratcher - shouldn't this work the way I'm expecting?