I just came across this as well. I'm running Wordpress 4.4.4 and Civi 4.6.4. I can add soft credits to individuals all I want but I cannot mix individuals and organizations soft credits in the same donation. Or so I thought, after messing around for a bit I found that if I removed the campaign that was set and the contribution page that was set (and saved the record). Then went back in I could add the organization soft credit.
I could then go back into the donation and add back in the campaign or the online fundraising page. This part was wonky and most successful when I added the campaign first, saved then went back and added the online page.
I was also successful mixing soft credits on a fresh donation when I only had the campaign set and no online fundraising page.
My take away, online fundraising pages in the set in the donation have something to do with it.