How do you extend the wires from the arduino board to both fsr sensors to be placed on both chairs (one for each)
In ADC sampling there will always be noise.  Longer cables may increase that noise.  Sensors with lower resistance tend have less noise.  For these reasons it would be better to start from a schematic, part numbers and / or specifications rather than a picture.

There are a number of ways to reduce noise.  Hardware solutions include twisted pair and shielded cables.  Software solutions include averaging multiple samples and removing outliers.

If all you are asking is if you can use longer cables, the answer is yes as long as you can tolerate the or compensate for additional noise if any.