Oh, I see. I have a valance on the box (divan?) part of dd's bed, so that it drapes down and hides the trundle, but the box is slightly longer than a standard divan so the valance doesn't hang well. Does the job though. I got it from John Lewis.
Our trundle is a full-length single bed, which is why the box is longer. The two mattresses are the same length. If yours is shorter, so that the top mattress is standard length but the bottom one slightly shorter, then a standard vallance should fit fine.
Having the valance under the top mattress means less laundry, and that all the children's fitted sheets are interchangeable.