Lets start with enabling and configuring flex fuel under Fuel Settings > Fuel Sensor Settings (Flex):

Most of this is fairly straightforward and obvious. Enable FF, use Flex input for sensor and fuel temp, use GM/Continental settings. I'd like to note the Fuel Multiplier here though. This multiplier is a linear scaling between the low and high values that applies to all fueling, similar to req_fuel (it might even apply directly to req_fuel), correct? Timing addition is the same but a fixed addition rather than a multiplier I imagine. Lastly, there is the Dual Fuel Mode which is configured in the Dual Fuel menu, so we will jump to that.

Table Choices > .Table Switch/Dual Fuel:

Table Switching vs Dual Fuel Switching is initially a little confusing, but I'm basically treating the former as a primitive version of the latter (for e85 at least). Using the Blend Curves in Table Switching for fuel and spark tables is the same as using Dual Fuel Mode except you can manually configure separate blend tables for each. Using Dual Fuel allows you to blend more than just fuel and spark tables and includes AFR, Idle, Boost, and AE table blending as well. However, it looks like all of these are blended using the same blend table (8) aside from the boost table (7) which can be configured independently using ethanol% or other vars, correct? In other words, Blend Curve (8) Flex Blend is used for fuel, spark, AFR, WUE, ASE, Cranking, and AE tables?

I really don't know too much about how the Table Switching side of things works though. Enabling some of these, such as fuel and spark, enables separate blending tables as opposed to the single Flex Blend curve used by Dual Fuel. You can also configure req_fuel and stoich switching, maybe for a non-lambda wideband sensor and the blend tables can be configured using axis' other than ethanol content. I am a little curious as to how when and why you'd use this option, but it seems that Dual Fuel should be used for most all flex fuel applications.

Now to tie it all together, final calculations when using Dual Fuel look something like this:

Fuel = (VE1 * (1 - blend8%)) + (VE2 * blend8%) * (lowFuelMult% + (ethanol% * (highFuelMult% - lowFuelMult%))

AKA VE1/VE2 blend via blendCurve(8) * fuelMult based on ethanol%

Spark = (IGN1 * (1 - blend8%)) + (IGN2 * blend8%) + lowTimingAdd + (ethanol% * (highTimingAdd - lowTimingAdd)

AKA IGN1/IGN2 blend via blendCurve(8) + timingAdd based on ethanol%

Boost Duty = (boost1 * (1 - blend6%)) + (boost2 * blend6%)

AKA boost1/boost2 blend via blendCurve(6) ! notice different blend curve, can mirror ethanol% !

Everything else = (table1 * (1 - blend8%)) + (table2 * blend8%)

AKA table1/table2 blend via blendCurve(8)