The only reason behind the split was to make sure there were 4 Old Firm games in a season for a 12 team league.
I'm still not a fan as, for me, a league needs every team to play the same number of games against the same opponents - otherwise you end up with MLB and franchises.
When we were in the old first division, there were 14 teams and we played everyone 3 times. It wasn't perfect, but they balanced the home/away schedules well enough and I still perferred that to 12 teams and a split.