Just detects the data columns for twin 1, and twin 2, then returns them stacked
on top of each other (rbind) with the non-twin specific columns copied for each as well.
*Note*, zygosity codings differ among labs. One scheme uses 1 = MZFF, 2 = MZMM, 3 = DZFF, 4 = DZMM,
5 = DZOS or DZFM, 6 = DZMF, with 9 = unknown, and then 50, 51,... for siblings.
Typically, OS twins are ordered Female/Male.