Hi, after reading the rulebook multiple times I have come to seek answers from the experts on this matter

The scenario in question depends on who is the first player, so I will lay out 2 scenarios which differs only in who is the first player.

This following sequence is followed regardless of who is the first player (at the time both investigators are at the same explored gate)

Investigator 1 is sucked into a gate without sufficient clue tokens to seal the gate once exiting it.

Investigator 2 comes across an elder sign while investigator 1 is going through other world encounters.

Investigator 1 makes it out of the gate and marks it with an explored marker.

Investigator 2 moves into the gate location to help seal the gate.

Here is the question: Can investigator 2 seal the gate with the elder sign once it is his turn in the Arkham Encounters Phase, or is he sucked through the gate? Reading the rulebook, it appears that only the investigator who made it out of the gate is protected from being sucked into the gate again, so my guess is: Yes, investigator 2 is sucked through the gate and cannot seal it with the elder sign (however the case can be made that this is a timing issue, if the elder sign can be used right away the gate would close before he can be sucked in. Or in fact some other rule I'm unaware of says all investigators are protected from being sucked into explored gates). If the answer is he gets sucked in once it's his turn, the first player starts to matter.

Assuming the ruling is he would be sucked through the gate:

Investigator 1 first player: The only option is to trade the elder sign to investigator 1 at the end of investigator 2's movement phase. Investigator 1 can seal the gate before investigator 2 takes his Arkham Encounters Phase.

Investigator 2 first player: Investigator 2 trades elder sign to investigator 1. Since investigator 2 takes the first Arkham Encounter, he would be sucked through the gate. Investigator 1 seals the gate, investigator 2 is lost in time and space.

If he wouldn't be sucked through the gate:

The whole discussion above doesn't matter and investigator 2 seals the gate with his elder sign at the earliest convenience.

Sorry for the long post, I could probably have explained it in less detail.. But I'm a bit tipsy, and I can hardly understand my own question

Basically, the explorer marker protects only the investigator having it. The rest is entirely up to the rules of the phases and who resolves their action first.

a) Investigator 1 moves before Investigator 2: thus,
Movement: 1 arrives and gets the explorer marker, 2 arrives and gives 1 the Elder Sign (and if he has enough movement points he can move away)
Encounters: 1 seals the gate, 2 resolves an encounter at the location (or elsewhere if he moved away)

b) Investigator 2 moves before Investigator 1: thus,
Movement: 2 arrives on the location with the open gate and must end his movement there otherwise he won't be able to trade with 2; 2 arrives and get the Elder Sign
Encounters: 2 is sucked through the gate, 1 seals the gate

For no reason at all, Investigator 2 is ever able to seal the gate explored by Investigator 1

::::::::::::::::::

If 2 is sucked through the gate, he's forced to explore the OWs until sent LiTaS. But it could happen that another gate to that same OW is open or can open while 2 is wandering in an OW, case in which he can return to Arkham when it's time, following the usual rules

Hope this helps!

Thanks for the reply!

So just to clarify a couple of things:

Trading can be performed at any time investigators share the same location? I.e. during movement when he has already moved, and moving again after the trade? I think the numbering of the investigators may be confused in the b) scenario ...

And it's only ever possible for the investigator who has explored the gate to close and/or seal it?

Yes, trading can happen at any time during the Movement phase only , meaning that you can move, trade, then move again, or move, stop, and then another investigator passes by and trades with you.

And only the investigator who explored the gate is allowed to seal it (unless something incredibly benevolent happens in the game, there's a couple of encounters allowing you to seal without exploring, but really, it's just a very very very rare case)