what dose cross check mean?

Answer 1

It means to double check but by checking against and not with. For example if you want to cross check if someone has locked the back door before you leave your house, you go in person and check if the back door was locked instead of asking did someone remember to lock the back door?

Answer 2

It means that when a F/A arms the door- attaching the slide- you look to make sure the F/A on the door across from you has successfully armed the slide on their door- and they "cross check" your door that you just armed