Hi. Unless the 2nd cat has specific health problems, I'd say there's no need to leave the food out for him. Feed them 2-3 times a day in separate rooms. That way you can give your 'big boned' kitty some diet food while the other one gets something with more calories. Just not being able to graze at the food bowl all day can make a big difference in the cat's weight. Your 2nd cat should have no trouble fattening up like this (as long as he's healthy otherwise), and you really don't want him to get into bad eating habits and end up fat himself.
In my case, I find that it's better to keep my overweight cats out in the main part of my apartment while they eat and lock the other in the bathroom while he eats. That way, I can pull up my biggest cat's food as soon as she walks away and prevent her from wandering back for seconds a few minutes later. In your case, you could leave the 2nd cat locked up (with food) for a longer time at first to help with the weight gain.