How to display posts together – PHP question

Im am wondering how can I create a custom post template, that will display two posts, for example one of the left and one on the right. AND how can I link the posts so that they will always be shown together. I know how to do the css but i can’t figure out the php.

Basically, on the left side of content area I want POST A, and on the left side I want POST B, but I ALWAYS want post A and B to be display together. And I want to continue that with future posts.

EDIT: if you know a plug-in for this, that would be cool too