what are the best ways if you have 2 2x8’s in an L to make a turn with flex track, specificly a double track main line, and how long rolling stock can you use
As stated, your question is a bit confusing. Where do you want the curve? If you’re going around the corner of the “L”, where do you want your tangent (straight) track to begin - in the middle of the 24" leg, at the far outside, somewhere else?
The length of the rolling stock will depend on the radius of the curve. I have a double-track main running on a 2’ wide around-the walls layout, and my minimum mainline radius is 36". I can run just about anything. My mains, however are close to the backdrop (a mistake, I now realize), which limits my scenery opportunities. So, it it sorta depends on what you want your layout to do.
Since I assume you have a point-to-point shelf layout , you may be limited to a mostly switching operation, so putting the mains against the backdrop would probably not be a good idea.
Again, we’d need a bit more information before anything definitive could be suggested.
Depends also on where you start the curve from. If you could fiddle with your approach such that the straight was already crossing from the back of the layout on one board and getting close to the front edge, you would get a nice wide curve if you were to add a triangular piece at the apex of the corner and use that surface for the apex of your curve. That would keep the next straight part of the track away from the backdrop area, as discussed above.
If you keep to the middle of the boards all the way, and have no wedge added to the sharp inside corner, your curve will almost certainly be 18" or worse/sharper (same diff if your engines are longish in the drives, or if you have already purchased longer passenger cars.)
If it were me, I’d add about a 6" high at the apex equilateral triangle to the inside corner and use that to broaden the curve out to 24" so that you can run a long steamer on there if you wish (we’re talking HO, aren’t we?)