after the first epoch of training a custom DDP model (1 node, 2 GPUs), two new GPU memory usage entries pop up in  nvidia-smi

You mean those two entries was not there during the 1 epoch, and only appears after that? What was executed after the first epoch? How did you initialize DDP? Did you specify device_ids or set CUDA_VISIBLE_DEVICES for the two DDP instances? Looks like both processes are operating on two GPUs. They should have exclusively run on a different GPU.