You should have a box for each cat, and you should keep them very clean.

Now that the damage is done, the only thing you can do is confine him to a
utility room or bathroom with his box, food, water, maybe a bed and some
toys, and get him back in the habit of going in the box. Cats are creatures
of habit.

Keep him in there for 5-7 days, go visit him and praise him when you clean
the
box. Let him out after a week and monitor the spot closely, as well as the
whole
rug. If he reverts, show him the spot, tell him "NO!" and confine him again
for
a couple of days. Repeat as needed. (This works for me, with a tuxedo cat
who
is doing the same thing.)

he's upset or he's sick
you don't need to retrain him

1. Have him see a vet to rule out a urinary tract infection.
2. Provide him with another box. He may want to pee in one and poop in the
other.
Gail