Depending upon the context, "being in each other's faces" can mean either that two people are in such constant close proximity that they have no personal privacy and everything done by one is known to the other.
It can also mean that two people are constantly arguing with one another.
"Becky and Darlene just don't get along, they are always in each other's faces," their mother sighed. "I can't wait until Becky goes away to college so that we'll have some peace in the house."
- For Teachers