No lines are plumbed together, but they mix somewhere. Faucets are two handle except tub is single. Is this where problem at & is it the cartridge?

Just what is the problem, you didn't give much info as to what was wrong.

Is it that your tub faucet is not mixing the water or it will not get the right temperature.

If you have single handle tub you likely have a mixng cartrdige or ball. A picture or brand name would help otherwise my directions may be way too generic to help you. What is the problem?