How can I make FastLED set every 2 LEDs to the same color?
The FastLED community appears to live at this reddit.com site. Consider asking your question there.
It is unknown if the following will exceed the current limitations of the Arduino platform you are using.  But, consider sending the same data to two different strings of LEDs simultaneously.  Briefly, each LED strips off the 1st packet of data and sends the balance of data through to the next LED.  Two different strips should behave the same if given the same data.  Thereby creating the effect you described (pairs of LEDs behaving the same).