I designed a split lying leg curl way back in 1982. I thought it targeted the hamstrings better than the Nautilus straight design. Recently my son, who is an exercise physiologist, said the design had to be alterred to more effectively work the hamstrings. His recommendation was to have the body in a raised, straight position with arms extended. I haven't reworked the design to incorporate his suggestions.
Either a machine correctly works a muscle or it does not. The split design makes the exercise easier for beginners but perhaps becomes less effective for advanced users. The test of any machine is how it feels when using maximum resistance.