Правило простое: each other обычно относится к 2 лицам или предметам, в то время как one another употребляется, когда число лиц или предметов, о которых идет речь, больше чем 2.
Molly and Jim looked at each other without a blush.
The engines were roaring and the two mechanics could barely hear each other.
The hostess and the guest understood each other perfectly.
They all were looking at one another.
You see, we – two clever people – have something to say to each other.
"He placed all the three candles next to one another.
The channels 39 and 45 are installed parallel to each other.
Then two trucks went by them, closely following each other.
They laughed at her like a family who love one another.
Eliza was my greatest friend at school, and afterwards we saw each other continually.
