Does your robot just have to go in a straight line? It sounds like going at a constant speed doesn't have to be a concern. Basically all you need is a simple encoder to keep track of your wheels' rotations. Typically you want to use two sensors per wheel for what's known as "quadrature", which lets you determine the direction the wheel is rotating along with its rotational speed. If your wheels are always only moving forward, you would just need a single sensor for each wheel.
Actually, if you really just have to go in a straight line on a flat course, you could mechanically link both wheels via a single axle and simply monitor the rotation of one of those wheels with a single sensor and an encoder wheel.