theoretically the two lenses have to be separated completely, can be difficult because of the synthetic cement. Then they are cleaned and recemented. The biggest problem ist: They have to be centered exactly. Only a very skilled repairman with the appropiate equipment (e.g. the CANON service) could do this. Another possibility would be to exchange the lens group. But I think the probability that they have a suitable and matched lens group for such an "old" lens as a spare part is very small and the price would be extremely high. Maybe someone here knows an independent repairman for separated lenses.