Complex sentence: A generation ago some neurologists would also spend a couple of years working in psychiatric units and obtain a Diploma in Psychological Medicine, but that became uncommon and now that a basic psychiatric qualification takes three years to obtain it is no longer practical.
Split into simpler sentences: A generation ago, some neurologists would have also spent a couple of years working in psychiatric units and obtain a Diploma in Psychological Medicine. However, this requirement has become uncommon, and, now that a basic psychiatric qualification takes three years to obtain, the requirement is no longer practical.