Assume that there are two balls left on the table (black and white). Can I call the pocket which is called by my opponent in his turn? We do not allow it, but we are not sure where the rule is coming from. I could not find such a thing on the internet.
The U.S. 8-Ball Rules do not contain any such rule; in theory you would have to read the entire rulebook to prove the negative, but Rule 6 "Call Pocket" is probably a good section to quote if you want to.
Obviously, if you're playing under a different set of rules, then refer to the rule book for whichever rules you're using.