Q: like two peas in a pod. In Dutch we have the expression (translated) "they are two hands on one belly" meaning that they always stick together. It seems that "like two peas in a pod" doesn't cover that. Is there an equivalent expression in English?

A: 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


A: 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.

