A good representation of a pit trap can be found in the
Hero Quest Sticker Album-Barrow of the Witch Lord page 25. While the first frame of the Elf falling in appears to support just one Hero in a pit, the next frame shows clearly how more than one Hero (or monster) could fit within. That second frame is fairly accurate as to the size of a pit trap (about 4 feet wide and perhaps 5 feet deep), as the Dwarf could fall in but get himself out without assistance as part of normal movement on his next turn.
I'm not sure where in the EU rules it is stated that only one figure may occupy a pit trap as more than one Hero may attempt to jump the pit on consecutive turns with a Hero within. I suppose Morcar players opt to observe the one-figure-per-square-space-at-the-end-of-a-Hero's-turn rule, where risk of falling in creates that situation if
falling into a square is judged equivalent to
moving onto a square. Page 15 stipulates that if a square is occupied (as it will be if a later Hero fails a pit-trap jump), a Hero must find another path or stop. So it seem to me if an EU Hero falls into a pit trap, this potential "illegal sharing of a square" blocks later jump attempts by other Heroes until he moves out at least two square spaces ahead or one square space back on his next turn.
If a Morcar player instead judges that a Hero within a pit doesn't block the way of a jumping Hero, then I'd say that's fine as long as the pit remains a threat where he falls in if the jump is failed (resulting in two Heroes sharing the pit). The other option for Morcar, allowing a Hero to step on the head or shoulders of a Hero in a pit and simply pass over is too comical for my Hero Quest game taste. Also, remember the Elf splayed at the bottom of the pit in the Sticker Book depiction--there's no stepping over his prone butt like a Hero-bridge, even if the Dwarf is tempted to.
Page 15 of the NA Instruction Booklet (read with an American or Canadian accent) explicitly specifies that sharing a pit square is permitted. This may strike some EU players that have imagined pit traps differently as an odd concept, but it avoids the need for further Zargon judgement.