Just to play devil's advocate
I don't think you've had anything but good comments and advice so far, however you could look at this an entirely different way...
Cary as many sizes, colors, and styles as you can possibly stuff into your pockets, vest, down the front of your waders, and bring a few extra fly boxes (BIG ones) in the car/boat "just in case". Try to judge which fly feels best based on some algorithm based, but not limited, on the following: fish size, run timing, water level, water color, water temp, run structure, weather conditions, wind speed, barometric pressure, time of year, day of the week, and the "mojo factor" (don't ask me about that last one, I have no idea myself...)
This is serious btw...
After tying on "THE ONE" fish it well, and fish it hard until it is no longer "THE ONE" then try tying on a smaller pattern, or a bigger pattern, a little more or less flash, etc. Repeat; fish carefully and methodically, knowing that the fly you have swimming through the water is THE best fly possible for the situation at hand. Keep this up until you hook a fish or two, then compare and adjust.
The important part of all of this is to find a fly that YOU will fish with ultimate confidence, which may change due to any number of reasons (some of them very legitimate, others not so much). I do think that the importance of focusing your time on learning to read water, and to cast and present the fly is key. Not wasting a lot of time fretting over fly selection, or changing flies incessantly, is good; but I don't think you need to limit yourself to just 2 patterns in order to due so. I think that the fly pattern (size/color/profile) can be a factor...sometimes... but there are far more important, and more controllable variables to worry about generally speaking.
Best of luck,