Pair
If you have one stool and another stool, you have a pair of stools. Look at this pair of stools. Look at it.
Imagine being in front of this pair of stools. Picture yourself sitting on one stool, getting up, then sitting on the other. Repeat this process.
そう
ふた
None
This pair of stools shares a soul (そう). Even if you tried to separate them, they'd eventually find their way back together.
It's the same with pairs of socks and pairs of people. Sometimes what looks like two things is really a perfect pair, with one soul. Meant to be together forever.