Lets just say both planets travel at the same speed to avoid collision and the other planet is on the opposite side of the sun?
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Two planets sharing a single orbit is not a stable configuration. Even if they did start off on perfectly opposite sides of the Sun, the slightest gravitational nudge (say, from a third planet in the solar system) will upset the balance and the two will gravitate towards each other, falling out of 180° opposition and eventually colliding. The leading hypothesis for how the Moon was formed is that the Earth collided with a smaller planet which shared its orbit very early in the history of our solar system, within a few tens of millions of years. We’re past four *billion* years without any collision now.
Also, we’ve sent numerous robotic spacecraft across the solar system that would have seen such a planet if it were there. The famous “Pale Blue Dot” photo of the Earth as a single blue pixel was part of a wider overview of the Solar System in which a counter-Earth would have been plainly visible. From Mars, Earth shines brightly in the evening skies like Venus does from Earth; any martian rover would have spotted a counter-Earth very quickly.
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