The plant that shoots one pea forward and one pea backward is Split Pea.
Split Pea is a specialized attacker from the original Plants vs. Zombies. It fires toward the approaching zombies in front of it while also firing behind itself, giving the player protection against enemies that slip past the main line. In the original game, its rear attack is particularly useful against Digger Zombies, which emerge on the house side of a plant formation.
The wording can cause a small mix-up because Split Pea’s firing pattern is not identical in every version or description of the franchise. In the original game, it is commonly described as firing one pea forward and two peas backward, but Split Pea is still the unmistakable canonical answer to this question because it is the plant built around bidirectional pea fire.
Threepeater fires into three adjacent lanes, Repeater fires two peas forward, and Peashooter fires one pea forward. None of those plants is designed specifically to cover both directions. Split Pea’s unusual orientation makes it valuable when a level includes attacks from behind or underground ambushes.