I like AT2020s. They are nice mics. I have a pair of the AT4050s, which sound similar. Both the 2020s and the 4050s have a little bit of a high freq bump. This will work in your favor as high frequencies will diminish a bit as sound propagates from the band to your mics.
The 57s wont be of much use. Neither the sensitivity nor feq response of the 57s make them suited to recording orchestra. However, you could set these up as a backup pair for giggles.
I would recommend using ORTF both days. If the smaller ensembles are on one day, consider moving the mics in towards the stage a little more so they fit the SRA better. Doesn’t have to be exact. Your concern about ORTF being too wide doesn’t make a ton of sense when the alternative you were looking at was XY. The SRA is almost twice as wide for XY as it is for ORTF. This is one of those counter intuitive things that people struggle with when they first encounter SRA. You would think ORTF, with it’s 110 degree mic angle would present a wider SRA than XY with mics at 90 degrees. But it doesn’t. Counter intuitively - the wider the mic angle, the smaller the SRA. Also counter intuitively - the further the mics are spaced apart, the smaller the SRA. If you get a chance, fiddle with the Sengpiel link. Start with ORTF and then see what happens if you depart from ORTF by increasing the mic angle (the SRA will decrease). Then change the mic distance and see what happens to the SRA.
By the way, if you don’t have one already, consider picking up an inexpensive adjustable stereo bar. Even something as simple as the K&M 23510 Adjustable Stereo Mic bar (which is only $20). You just need to be able to adjust mic distance and mic angles. If you have a protractor, bring that along too. I picked up an inexpensive one at Lowes. You can use it to set mic angles, and you can use it to help see the SRA.
Question of 2 mono vs 1 stereo file - I don’t track bands with Cubase when I’m in the field. I use dedicated hardware recorders - a Sound Devices 788T is my primary recorder and I use a Roland R-44 for backup. I typically record to one file (poly) then tear the file apart into separate mono tracks when I import to Cubase. If you’re recording directly to Cubase, I might go for a stereo file, rather than 2 separate mono files, as you’re drive is only saving one file at a time, not two. Although your computer can probably handle 2 files fine. If you recorded a stereo file, you can always pull the file apart into 2 separate mono files later if you need to. Panning for ORTF is where the left mic goes hard left and the right mic goes hard right.
If you have a dress rehearsal, definitely record that if you can. It will help you figure out levels. And…if the concert recording has a problem, all is not lost as you at least would have the rehearsal files. Besides - you can have a listen to your recording of the rehearsal and make adjustments for the concert. I have this same scenario with a jazz big band I’m recording in two weeks. I am going to record the rehearsal the night before, then record again at the concert. One of them is bound to turn out OK!
-Tom