like two peas in a pod.
Solution 1:
joined at the hip TFD
spending a lot of time with someone else, often a close friend
As in:
Every time I see Jenna, Kelli is right there with her. Those two are really joined at the hip these days.
Solution 2:
You don't even need an idiom (unless you want one); there is a simple English word that means the same thing.
Inseparable:
2 : seemingly always together : very intimate · inseparable friends