Although we don't have a considerable experience in lengthening procedures of congenital forearm stumps, I would prefer lengthening of the both forearm bones, whenever there is a possibility for that.

Dear Dmitry

I have no experience but this article may be useful for you:

Prosthet Orthot Int. 2003 Aug;27(2):153-7.

Lengthening of short stumps for functional use of prostheses.

Orhun H1, Saka G, Bilgic E, Kavakh B.

Abstract

The length of a stump plays an important role when using a prosthesis. It is essential to have an adequate length in the stump in order to apply the prosthesis. Therefore, it is necessary to attempt to lengthen short stumps. The authors have reviewed the results in 2 patients who had such lengthening of stumps one due to traumatic amputation of the forearm at 6.25 cm distally from the elbow and the other due to congenital deficiency of the radius and ulna treated by the Ilizarov techinque to improve the fitting of prostheses. Sixty percent (60%) and 40% lengthening was achieved respectively. At the end of the lengthening process, sufficient stump length for the prosthesis was achieved. There were no major complications. As a result, it is believed stump lengthening is a valuable method for cosmetic and functional use of the prosthesis. The Ilizarov Technique is an effective method of lengthening of a stump and it does not require a tissue expander in selected patients. Patients have been able to use their prosthesis 6 months after surgery

Dears colleagues. Thank you for information and publications. We find logical to lengthen both bones. But in literature another opinion is also presented.