The conditions are that you have to get both endings on a single fresh save file without using the cheat code or developer mode. However, you're not allowed to ever enter the village, either through the A door or through the back entrance found in the trophy room. Furthermore, one of the ZUG doors has to remain locked.
Personally, I had to look up the winner's playthrough to learn the location of (don't read this before you're done with this challenge)
Spoiler: (click to show/hide)
the secret room in the O area with the master key as well as the illegal gun shop in the L area. Like him, I decided to leave the U door locked and obtained the gold trophy with teleport boxes. However, I retrieved the silver trophy by throwing a teleport gun against the ceiling. I screwed up several times, so I had to visit both illegal shops multiple times to stack up on teleport guns and boxes as well as three item boxes.
Was pretty fun overall, though some parts were a bit too frustrating and grindy. Though in hindsight, there would've been less annoying alternative solutions to these sections.