Yes, alternating heavy/light DL is fine. I do this often with trainees. I often lower the load anywhere from 5-20% from the heavy day and increase the volume a bit to 2-3 work sets. Start with a 10% reduction in load and perform 2x5. If it seems too easy then either reduce the % offset a bit or add a set. If it's too hard then increase % offset to 15-20%