Also what time - there's a significant difference in peak/off-peak fares on this route.
But as a basic guide, splitting at Doncaster (all tickets below are off-peak day returns) you save £1.10: York-Barnetby £18.10, compared to York-Doncaster (£7.85) + Doncaster-Barnetby (£9.20), total £17.05.