I'm still a newbie really to Tableau Prep but as far as I know the splitting of fields works the same as Tableau Desktop (so either automatic or custom)
I wonder would a cleansing step help?, as described in this thread...
cc'ing Maraki Ketema in case she would like to add further inputs
Kind Regards, Ciara
Ciara is pointing you in a good direction - the split will create 2 (or more) dimensions (columns) that you want to convert to a single column - you would pivot the data after the split