Is there an option that allows a multiclass druid/summoner to combine his animal companion and his eidolon into one creature?
Animal Companions come from a list of animals, and while the list is not all inclusive, notice:
Druids and rangers have a wide selection of animal companion choices, but this selection by no means covers the entirety of animals available as companions.
Eidolons are by definition outsiders:
A summoner begins play with the ability to summon to his side a powerful outsider called an eidolon
You can't have both the outsider and the animal keyword on a monster or being, they are mutually exclusive, especially noticeable when you get to lvl 20 in one of several classes (like monk) and change your creature type from whatever you were before to native outsider. This would be the pinnacle of what the animal could achieve, but then it instantly looses the ability to be an animal companion, analoge to what happens if you use awaken on your animal companion:
Its type becomes magical beast (augmented animal). An awakened animal can't serve as an animal companion, familiar, or special mount.