I have never heard "tied at the hip"; however, there is a common English idiom "joined at the hip." This means that two people are together all the time, have the same interests, think the same way about things, etc. It's an idiom that is usually said in a rather critical way, e.g., "Don't bother asking Mary if she agrees with Laura--those two are joined at the hip!"

not capable of being separated

inseparable

it means If two people or things are joined at the hip, they're so closely linked as to be almost inseparable. e.g The two clubs were so closely tied after sharing the same ground and the same staff for so long that they were practically joined at the hip. cowboy...