If your two signals exceed +/- 0.5 before summing by addition, then the result will exceed 1. The [clip~] object allows you to specify upper and lower ranges, [tanh~], [sinx~] and others will do folding at the limits, but the simplest way is to first ensure each signal is less than 1. then divide each by 1./2. or 0.5.
But what you're looking for is called a limiter. That will attenuate signals over a set threshold with variable severity. [clip~] would be the upper extreme, also called brickwall limiter. Check out [omx.peaklim~] for an adjustable one.