Composite keys

Excuse the noob question, I couldn't find any reading material on this
topic.

Let's say my_table has two fields, pkey_id and another_id. The primary key
is pkey_id and of course indexed.

Then someone adds a composite index on btree(pkey_id, another_id).

Question 1) Is there any benefit to having pkey_id in the second index
(assuming the index was created to satisfy some arbitrary WHERE clause)?

Question 2) Regardless of the answer to Question 1 - if another_id is not
guaranteed to be unique, whereas pkey_id is - there any value to changing
the order of declaration (more generally, is there a performance impact for
column ordering in btree composite keys?)

Thanks,

Carlo