Use "fill left" audio effect for channel 1, "fill right " for channel 2.
I use audio track duplication sometimes to split both left and right stereo audio out to separate mono tracks. There is also a global setting for how Premiere should interpret stereo audio, single split channel track or dual mono track.
Yeah i know how to perform the function I was just wondering if you should be having two audio tracks; what the benefit would be of having two tracks of one mono source...
Were I doing a 2-channel Master, I would Fill to both of those channels, unless you want the signal to only come out of one speaker.
In a 5.1 Master, I would keep that lav Audio in mono, and locate it to Center. Depending on the Audio, I might Duplicate it, then Fill for a 2-channel Stream, and locate a little to the L and R, attenuating those by maybe -6 to -8dB. Still, for dialog, I would first want the mono signal in Center, but that is just me, and I might change my mind, depending on other aspects of the Audio in the Project.
It doesn't really matter much in production, so long as it comes out right in post.