The one horizontal line indicates that two people keep in step with each other.
Its big seal script is
the two lines emphasize the same pace.
Its small seal script is
the lines are not continuous but the meaning is the same.
Its clerical script is
which put two people in one body.
The two people on the top become two dots, which is not good change.
There were 並 （Post 241), which means "Side by Side". In the simplified Chinese, they are merged into 并, which means that 並 is not used in simplified Chinese. Therefore, 并 means Abreast (walking together) and Side by Side (standing together) in simplified Chinese. Traditional Chinese keeps both in different meanings.