If you google that question as is, you get a plethora of links and "how to's" on how to do it.
This and this or this, are practically useless. They are badly written, some steps are "missing" (like how do you go from Forwarding and POP/IMAP to so called "Pop Download"), and I think they are out of date as the interfaces and options in Gmail settings look different (as of Nov 2015).
It is obvious the methods explained are about "opening up" Gmail accounts to POP3 connections and treating the other Gmail account as an external POP3 account.
But it does not work for me.
Every time I give authentication to the POP3 settings of the "other" Gmail account, I get authentication denied errors. I've checked the Gmail passwords and access to both accounts on a separate browser and it's all OK.
I am trying to do this in latest Chrome, Windows XP.