I presume he means something like a two pixel piece of a LED strand folded over so that each LED shines in the opposite direction. Now to use the six bottles as one strip would mean running the data from the first back along the pendant cable (4 wires) and into the next. This would also mean a 330 Ohm resistor in series with the "DIN" of each two pixel piece.
The original question is whether the FastLED library facilitates treating pairs (or larger groups) of LEDs in the chain as single "pixels". I myself do not know having not used it. The alternative is the Adafruit library and I don't know for that either.
Now if you are totally sure you never want to treat the two "back-to-back" pixels separately, then you simply connect the data in to both "DIN" in parallel and take the "DOUT" from only one of them to the next module. 