Oh guess what, I just spoke to another "reputable" supplier. Their sales rep said they couldn't help me because they didn't sell twin cartridges but only singles. Argghhh.
Edit: They just called me back for the 2nd time. The first time was to tell me their technician confirmed they can't help me. Now they have left a message saying they definitely can help me. I'm going to call them back as I'm now dying to know what they could possibly say. Stay tuned.