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Please answer the following so I can help...
1) How old are the two cats?
2) Male, Female? Spayed?
3) Has there been ANY change in their daily routine, food, environment, your schedule, etc.?
4) Have both cats lived together and if so, for how long?
5) Are they both 'equally jealous' of each other?
6) Is there any other behavior or signs that do not seem to e normal for either one of them? Eating, potty habits, other behaviors, etc?
Thanks for the helpful information.
The first thing you will need to do is rule out a medical problem. Many times, when there is a sudden behavior change OR problem behaviors increase, an underlying illness is a contributing factor. You may not see any symptoms of illness because cats can hide these things very well - and some illness do not present themselves in ways we can easily see.
Before we can look at behavior modification or trainiing, the will need to be ruled out because if it is an illness, no matter what we try will not work. Please schedule an appt for your older cat for a full examination including blood/urine tests.
I wish you well,