So one player is on the crown of command, the other on the cave of the demonist.

the second one rolls on quest number one, which says he has to take the life of another player.
This one can't do this, as the only other player is on the crown of command AND following the rules he should have to complete the quest of the demonist before he would be able to have access to the portal, right? so another talisman isn't enough, but he can't get to the other player to complete his quest.

Does this mean the game ends by the one player "crowncommanding' the other who's just running around (or giving up?)

If I understand you correctly, the scenario is as follows: In an attempt to get a talisman, a player took a quest from the Warlock's Cave (which is what I believe you are calling the cave of the demonist) and the quest is to take a life from other player. But, the only other player playing the game is already on the Crown space. So the first player cannot enter the Crown space because they have an uncompleted quest and are therefore unable to pass through the Portal of Power. If this is a correct understanding of your scenario, then yes, that one player is pretty much out of luck.

Although it might be possible to get around the situation (like defeating the Lord of Darkness if you have the Dungeon expansion),the options are few and time consuming, that you may as well just give up.

Gelmaron said:

You can also use the transportation spell and switch places with the other player.

Carrion Prince said:

Which is pretty hard, as you have to be in the same region as the char you swap with. If the other char is on the Inner Region, with the "take a life" Quest, you can't get there, SOL .

Dam said:

There are there ways to enter the inner region if you beat the boss in the dungeon. If you win by 6 you go to the portal of power. If you win by 7 you go to the plain of peril. If you win by 8+ you go stright to the crown of command. If you have the transportation spell you can swap with someone on the as long as you beat him by 6 points. This is pretty hard of course, but not impossible.

Carrion Prince said:

Portal of Power is on the Middle Region, so no dice there. Only result that lets you use Transference is 7 (since at 8+ you're at the CoC). What then? You take a life, then get bounced to the Warlock's Cave? That's what should happen per the rules, you complete the Quest, poof, to the Cave with you.

Which is one of the reasons I'm a proponent of "no access to the Inner Region in any manner if you have an uncompleted Warlock's Quest". Probably alone in that sentiment, but oh well.