We have a 9 month old, female and an 18 month old, female cat.  They
are both fixed and not related but they get along fine.

The younger cat is very attached to people but me in general.  I have
never seen a cat or kitten that wants to be with and on people as much
as she does.  At night and in the mornings she licks my face and nips
at my chin.  She sleeps at my side or on my back and follows us from
room to room.  She is an extremely loving and cuddling cat.  She is
also very vocal and is 1/2 Siamese.

Last week, we went on a 5-day trip and left the cats alone in our
house.  A friend came by twice to give them food, water and clean out
the litter boxes.  The first night we were gone, the younger cat
pooped (diarrhea) in 4-5 places behind the furniture.  She had no
health problems and the problem did not continue once we got home.

We typically work 12-14 hour days and the cats are home alone during
the day.  I'm thinking she had the biggest problem at night since she
typically sleeps with us and might have gotten stressed when we did
not come home.  We have gone on shorter trips before and did not have
any problems except one time when she had a similar occurrence but to
a much lesser degree.

What can we do to help to avoid this behavior?   Is this common among
younger cats and do they sometimes grow out of it?

Thanks for the assistance.