[theWeezL]

If your talking about the do-luck center bar, (the one that follows the floor just in front of the rear seat) from what I read it wont do alot for twist, its just not designed for that. It does have some resistance to twist thought so long as you actually use both bolt positions (alot of people dont) because then the bar acts a little like a torsion bar. Would both work? yeah, in combination they would be great for stiffening up out Targas.

The targa bar is meant to do two things:
First of all, look at your car from a profile. Essentially its a big flat triangle. When you remove the top you are basically chopping the top off a triangle, making it no longer a strong structural shape. The front and back sides of the triangle are going to want to come together to complete the shape again and the only thing stopping them is the rigidity of the bottom side. So, Im filling that gap again.
Second, take a box with a lid on it...a nice flat rectangular one works well for this demonstration. Apply a twisting force on it from corner to corner. It should be pretty resistant to that force. Now take the lid off of it and try it again...not as resistant. Thats because a rectangle is basically just two triangles. So your lid is resisting the torsional load.
Side note: if you fabricate a bar that crosses the center line of your box...just how much do you think its going to do to resist either of these forces?

And dont be too quick to condemn the bar because it approaches T-top...give me a few hours to clean it up, prep for paint, and get a couple coats on it...I WILL have pics today sometime