I have 2 male cats that are both in and out door cats. Whenever I go away for a few days I leave 2 litter boxes in the house for them to use. The 8 year old more dominant male will use both boxes. The other one who is 12 yrs. old will not use either and instead will just eliminate in the house. My guess is he won't use the box because of the other cat's smell. Other than separating the 2 in the house, any ideas on how to overcome this problem.