I have a late 2011 MacBook Pro 13.3 in. with 4 GB RAM. I want to upgrade to 8 GB, and all the websites I have looked at to buy the upgrade seem to send it with 2 x 4 GB chips, so I am wondering if I can buy 1 more 4 GB chip and just put that in and have 8 GB or if the system doesn't work that way, and even though there are 2 4 GB chips they are somehow connected, so you have to buy it as an 8 GB upgrade?