When enabled the Polyphonic Legato option found on most of our legato interfaces allows the user to play 4x monophonic lines within one legato patch.
It does not create a full Polyphonic patch. Instead the Polyphonic Legato divides the MIDI note velocity range of 1-127 into 4 bands. One band per legato voice. The velocities split is as follows:
The velocity you play a note at determines it's 'voice' number. Notes with the same voice number are considered part of the same legato.
This means you could write one line of legato with the velocity '1', and then overlap, on the same MIDI channel, another line of legato with the velocity '33' and have two simultaneous legato lines on one MIDI channel, without them interfering with each other.
Screenshot showing where to find the Polyphonic Legato option: