It depends, Romanian deads ( Stiff legs) are lower back, glute, hamstring work, so they can be done on leg day, and not interfere with your back session.
If your doing sumo, or american deads, ( deep leg involvement, ) you will handle more weight through a deeper range of motion, and your quads and traps will get more work, as well as the muscles that run across the entire length of your spine. In this case, put your legs, and back work together since this exercise will be for both. E