Not sure why you would need two timelines - the export format is determined when you export, not when setting up a Sequence.
Also, there is no need to render the timeline itself usually, unless the effects are so heavy that you can't get smooth playback for a segment you wish to review/check results of. You can leave parts of the timeline red or yellow and when you export, the source video is rendered direct to the output format.
When you render previews, and also check the "Use Previews" button in Export settings, you are adding a generation of compression since you are taking the rendered preview clips (temp clips) and compressing them yet again.
Many former FCP users render the timeline out of habit, not necessary with Premiere most of the time though, please see -
http://blogs.adobe.com/premierepro/2011/02/red-yellow-and-green-render-bars...
Thanks
Jeff Pulera
Safe Harbor Computers