I am also looking for a definitive answer on the question you are asking. My experiences with SNDS lead me to believe that separate accounts would be best.
Here is my setup:
I run multiple websites (different domains). Each website runs on a dedicated ip and sends a newsletter from that ip. Those ip's are added to 1 SNDS account.
I send the newsletters typically on different days. For example: website 1 on monday, website 2 on wednesday and so on. My ips have been green for months.
Also note that a reason for a yellow ip can be that your sending schedule is altered. snds support confirmed this to me.
So last week I sent the website 1 newsletter on monday. (IP=green) Then on wednesday I sent the website 2 newsletter. Due to circumstances I had a lot of new content on website 1 that I wanted to share with my subscribers so I also sent an extra newsletter for website 1 on wednesday. The result was yellow ip's for website 1 AND website 2.
This could ofcourse be a fluke. Or something related to the content of the newsletters.