Is it possible to use a splitter instead of a quad LNB?
I am afraid not. There is upper band and lower band, and vertical and horizontal frequencies. Each of the LNBs in your dual LNB has to be set on one of these 4 combos there.
Your DStv kit is telling the LNBs to go to lower band, with one on horizontal and the other on vertical. You need that separate LNB to go to upper band.