Personally, i think that if Penny is 12 years old and is very attached to you, it would be very hard on her if you left her behind - unless you plan on visiting her every day. my cat is like that, and i could never bear to be separated from him. and especially since Penny is 12, i would not leave her. it doesn't sound as if she and Copper will suffer from separation anxiety, but i guess you never know until they are apart. if that is the case, i would take them both, but i would never leave her behind if she is that attatched to you. this is just my opinion

good luck in whatever you decide.