The two-way crab is an intelligent alien with a strange form: It is biradially symetric, with multiple legs at the sides, 2 arms beside each face, and two faces at opposite ends of the body. It has no defined head. Each face has eyes, other sensory organs, a mouth, and a small internal ganglion. The brain is in the centre of the body. It has no shell or armour. Its body is roughly as wide as it is long. It can move in any direction with the same ease. Would this bodyplan work, and is it plausible that it could have evolved?
Radial symmetry is most commonly observed in organisms that move very little, if at all. The radial symmetry allows the organism to sense and interact with stimuli on any side of the body, without needing to move the body itself. For an intelligent, mobile creature like a crab, having a second face likely won't bring much benefit - since it is not a sessile creature, it can just turn to face nearby stimuli.
Body complexity is a liability, and is unlikely to arise unless there's a evolutionary benefit to doing so. What's the benefit to having a second face? It opens new ingress points for bacteria or infections, and the benefits are unclear. The crab can likely only feed one mouth at a time, and a wider range of vision can be achieved without needing a second face.
Given that there are currently no known creatures that have two faces, and very few mobile creatures that are radially symmetric, the natural evolution of such a creature in an earth-like setting seems unlikely. To make this more plausible, it should be clear what's the benefit of any of these features like a second face that cannot be achieved with just one.
In case of two sets of eyes with a single brain, you could have a viable crab.
But I see a few other issues..
there is two vulnerable parts instead of one, predators would finish them off easily.
the eyes would be very primitive, because the connection to the retina will need to be relatively long. Normally the retina has a very short connection bundle to the brain.
internal competition between the two sets of eyes could arise, the animal would end up having a split brain, the motoric system will get disoriented.
no idea what happened on other planets, but on earth, animals developed from a tube shape, one side to take in food, the other side to get rid of the excrements. Having two mouths on either side would violate the basic design. The mutation would need to be huge. A second mouth would require developing some outlet elsewhere, at the same time ! The probability of two rigorous mutations taking place at the same time is very low.