This is the 5149th most frequent Turkish word.
ikisini
"Both of them" or "the two of them."
Here, 'ikisini' refers to 'two of them' indicating the object of the action.
Kitaptan ikisini aldım.
I took two of the books.
In this sentence, 'ikisini' means 'two of them' specifying the individuals invited.
Arkadaşlardan ikisini davet ettim.
I invited two of the friends.
In this usage, 'ikisini' highlights 'two of them' referring to the pens separated for you.