I have to separate my ducks for mealtime. Two eat one food, the third is on a special diet to prevent egg laying. The entire process takes about 10 minutes. We have a section of their aviary fenced off with a little gate. Two are trained to go in, we open the door and they go right in. They each have a water bucket and food near each other on either side. They are completely comfortable with the process, even though they are a bonded flock.
If you choose to separate them, I would put Fatti in the new space for mealtime, not the other bird, and have their dishes as close to each other as possible. Be aware Fatti might try to eat through the wire so the other bowl might need to be a little further away!
Birds can have health problems from being overweight, and if you really can’t find the keel bone I would take some steps to limit overeating if it were my bird and an adult. I would also check on the weight of the other bird to make sure she/he isn’t underweight.
Stress can also have real health consequences, so think about how much separating them for feeding is upsetting them. One of my birds was going to die if we didn’t – so for us it was worth it. And they adjusted quickly!
Edited to put it “an adult.” I missed that the first time!