I don't think that's what he meant. I think Seph's script merely changed the party member graphic on the map, like it scrolled from Aluxes to Basil, etc. What Persistent wants is for two parties (or more) to be on the map at the same time, and you can move both of them, switching back and forth with the press of a button. I made a similar request on RMXP Universe, and Dargor said he would add the effect into his Party Changer script, so look out for it.
There's also
this script by RPG Advocate, however its functions are limited; you can only have 2 parties, and 1-3 members (has to add up to 4) in each party.
Hope this helps