Unless this is a poison specifically meant for killing eidolons, he should be fine once he dies and comes back to life. Or if you want to be a good friend to your eidolon, just dismiss it until you get the capability of curing the poison.
Eidolons are not destroyed when their physical forms die. Their "soul"/"Spirit"/whathaveyou simply returns whence it came. The physical form you interact with when you've summoned your eidolon is simply a shell which it is possessing, maintained by you the summoner. Which is why you can fluff yourself as a cultist summoning the avatar of your god, yet said "avatar" only has 6 Hit Dice at 8th level.
From the Unchained Summoner as per the PRD:
A summoner can summon his eidolon in a ritual that takes 1 minute to perform. When summoned in this way, the eidolon's hit points are unchanged from the last time it was summoned. The only exception to this is if the eidolon was slain, in which case it returns with half its normal hit points. The eidolon does not heal naturally. The eidolon remains until dismissed by the summoner (a standard action). If the eidolon is sent back to its home plane due to death, it cannot be summoned again until the following day.
I will assume that despite the awkward phrasing, it is the summoner, not the eidolon that is dying of poison.
Then, if I understand, the OP wants to play 2 level 4 summoners, instead of a level 8 summoner, but have the two level 4 summoners be considered the same as a level 8 summon as far as their eidolon, which they will share.
Frankly, this isn't at all supported in the rules.
The eidolon part is of course balanced, since it is exactly what a level 8 summoner would have, but 2 level 4 characters aren't the same at all as a single level 8 character.
In some ways they are stronger, particularly action economy. In other ways they are a lot weaker, much lower saves, spells and all that. For most classes I would say 2 level 4s are a lot weaker, even with the action economy. With the combined eidolon though I would guess that this combo is actually a bit stronger. The eidolon will still be the main combatant, but will have twice as many actions from his 2 minion summoners to support him.
In any event, as a GM I wouldn't allow it, partly because of the uncertainty of power level and partly because a single character with a pet is already taking up a good share of game time, adding in another character doesn't seem like a good idea.
I will assume that despite the awkward phrasing, it is the summoner, not the eidolon that is dying of poison.
That makes more sense.
I agree... There is no way that I know of to accomplish a pair of weaker summoner's maintaining a single powerful eidolon. Nor would I allow any form of house rule to do so at my table. This kind of outside-of-the-box character design isn't what pathfinder excels at, and it would cause too many imbalances in the rules to be allowable.
Best way to go about doing this would be 3rd party rules, 3rd party race Dvati from the dragon compendium. It deals with "twins" and how they deal with action economy, hp, saves etc.