Hi jdvk,
I’m not Diderot of course but they did mention what they purchased in the first part of their post …
Off the top of my head … some of the local options that have (or at least may have) side to side split mattresses include …
http://www.roomandboard.com/ (their natural latex mattress)
http://naturalsleepandhome.com/ (OMI makes some mattresses that have side to side splits but I don’t know if they carry them)
Redirecting... Savvy Rest latex mattresses are all available with side to side splits)
http://americasgreenstore.com/ (Some of the Suite Sleep mattresses have side to side splits but again I don’t know if they carry them)
There may be others as well that I’m not aware of.
The first part of post #2 here also has more information about accommodating two people that have different needs and preferences. Some mattresses have a design that can work well for two different people and may also make a good choice but of course you would need to test these to make sure that they are suitable for both of you.
Phoenix