After importing the mixes the first thing to do is to assign the clip the appropriate output channel. To do that we have to select the clip in the Event Library andwe select the channel that clip should have in the Audio tab of the inspector panel .

We are gonna use the Roles to be able to separate the Surround mix from the Stereo mix when we export so we are gonna have to create them in the info tab of the inspector.

We create a role for the 5.1 mix and another one for the 2.0 mix (I also created subroles for each channel but they are not needed for this task).
When all clips have the audio channels and roles propperly assigned we can add them to the project.

In the Timeline Index the new roles show up and we can toggle the mixes on and off for monitoring purposes.

Then we can proceed to export the file.

When the export window shows up we choose "Roles As Multitrack Quicktime Movie".

Then we are able to chose which roles are gonna be exported. Here is important to choose the right order as that will be the order of the tracks in the exported file. We add an Audio track.

Then we choose which role is gonna apply to that track.

And we make sure is the right audio format (in this case is the 2.0 mix so it should be a Stereo track).

After all this is done we can proceed to hit "Next" and tell FCPX name and location of the exported file.
This is gonna gives us a quicktime file with 8 Audio tracks. The first 6 correspond to the 5.1 mix following L, R, C, LFE, Ls and Rs. And the 7 and 8 tracks will be the 2.0 mix with L and R stereo tracks.