Your SmartFade console has two rows of faders. When the console is started in Two Scene Mode the top row is connected to Crossfader A, and the bottom row is connected to Crossfader B.
In other words, it is the number of physical faders of the console that will decide how many channels your console can control in this mode.
Smartfade 1248 and 1296 = 12 channels
Smartfade 2496 = 24 channels
Preparation & crossfade
Step 1: Make sure the Grand Master is at full, and that BLACKOUT is off.
Step 2: Make sure Crossfader A and B are both down. This means that light is being output from the bottom set of faders since Crossfader A is at 100%
Step 3: Set the channel faders in the top set of faders to new values. No change is visible on stage yet, because Crossfader B is at 0%.
Step 4: Now move both Crossfaders to the top position simultaneously. The output will change from the levels of the bottom set of faders, to the levels of the top set of faders. The time used is the default time shown in the display. You can change this with the wheel anytime.
Step 5: Now the top set of faders control the output. This means that you can set the channel faders in the bottom set of faders to new values.
Step 6: Crossfade to these values by moving both Crossfaders down.
Step 7: Repeat from step 3.