414(b) and (c) state that all organizations which are members of a controlled group are treated as a single employer for the purposes of sections 401, 408(k), 408(p), 410, 411, 415, and 416. So I think that for purposes of annual additions, the compensation from both companies would be included, regardless of whether the second company adopted the plan. For the purposes of the deduction limit, you would only count the compensation in the company that adopted the plan.