Why does Audition create two seperate channels when I import .omf from Premiere?

While it is possible to create a stereo OMF (I think Steinberg, Sonar and Media Composer may/can create one) it is advisable to NEVER try.
The reason is quite simple - when you send out a stereo OMF you have to be aware of who is going to either open it correctly or even open it at all?
Audition won't open it and neither will ProTools or most other OMF capable DAWS.
So everyone sticks to mono if creating an OMF and stereo is usually not even an option
Now for those that don't know AES31/ADL was originally created as a specification for universal conversion between DAWs, however, AVID essentially ignored it in favour of their own OMF1 which was said (by them) to be superior.
Of course it wasn't - it was the most convoluted thing ever devised which prompted OMF2 which is the current 'specification' used by Audition and others.
Needless to say OMF2 (with its 2gb limit) was replaced by something even more complex - AAF.
As an aside, the unnecessary complexity of OMF and AAF (and also MXF) make them a "use at your own risk format."
Although there are other formats that can be in the same category, these three are the worst.
They are self proclaimed standards that never should have been created.
Although the container format is always a standard, the methods of describing data within the container are not.
They each have multiple ways of describing the same thing and since everyone has a slightly different way of doing things within the containers, they might as well not exist at all.
To call any of these formats any kind of standard is a joke.
OMF is literally a format within a container format.
AAF and MXF are much worse because they are a format within a container within a structured storage format.

In case you hadn't noticed these are not my favourite formats 😉

BTW Audition's OMF implementation is second only to Avid/ProTools

As far as I can make out, this is an OMF format limitation - it's been complained about at lot on multiple DAW platforms. Hopefully SuitSpot will pick this up - he'll be able to give you chapter and verse.