How does commercial yeast contribute to dough consistency?

Welcome to TFL.

I noticed that when I use only sourdough starter as the proofing agent, the dough becomes much less cohesive and was flapping around much more than, when I use sourdough starter + 2g commercial yeast. Any ideas?

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I'm guessing that the issue is either: 

A) you're using less sourdough starter/levain when you add dry yeast, so the sourdough is having less of an effect.   ... or ...

B) when adding dry yeast, it ferments/rises quicker, so that the starter/levain has less time to have an effect.

another thought: if the dough + starter/levain is too floppy, maybe you're using too much, or your starter/levain is too acidic or old/degraded/proteolytic due to feeding schedule or feeding ratio.

 I'm not that good at diagnosing starters.  But if you can describe your starter/levain feeding schedule (timings) and feed ratios, and overall formula (recipe), the starter experts here might chime in.

I have 2 baking stones, one on top of the other few racks up and I heat them both so to mimic professional oven but it seems that the upper stone heat (going downwards) is caramelizing the top baguette surface too quickly? I'm using steam at the beginning of the bake.

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Yeah, the upper stone is likely causing the problem. It's _radiating_ heat back down at a point in time when you definitely don't want it.  And it is too close to the bread compared to a real brick oven.

"Upper heat" is a common topic among newcomers here.

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You are SO in luck!  An excellent thread on baguettes has just been done, and even has the successes summarized in PDF form. http://www.thefreshloaf.com/node/64622/community-bake-baguettes-alfanso

The post goes just over 7 pages, over 2100 comments, so Alfanso created a PDF summary, see the link near the top of the post. It's a really great tutorial.

Benito (Benny) just created a series of videos, if you are more of a visual learner. http://www.thefreshloaf.com/node/66116/baguette-baking-videos

Good luck, and bon appétit.

If the dough with only starter doesn't come together as they say (which is really a lack or destruction of gluten), you probably have a starter issue ie it became too acidic. 

Take a very small amount and feed something like 30 to 1 and

Keep warm

Stir ever 12 hrs

Don't add food

Wait till it starts growing again - give it a few days, maybe more.

Once it comes back go to a normal 122 daily feed. Enjoy!