Marc, I think that what you want may possibly be hazardous for people with other workflows.
If I understand you correctly, you have one camera and one card, and two computers.
You start taking photographs, and download them to the first computer.
Then you take a second set of photographs, and now want to download only the second set to the second computer.
If Kirk would make something that would automatically prevent the second computer from downloading the first set of photographs, then people who, contrary to you, do want the second computer to receive ALL images from that card, will have a big problem because they may reformat a card with has images on it that they haven't downloaded to their computer.
In your case you either have to download the images manually, or download too many of them. They can easily be trashed, so you only lose a bit of time.
Wouldn't it be easier if you used two separate cards, one for each computer and clearly marked?