2 columns needed

Hi!
I read the previous posts about 2 columns, I tried to follow them, but with no succeed!
I am using a free demo template, that already has the option to choose 2 columns for posts.
I want to add borders to posts, but if I do so, I don’t have the 2 columns anymore, but only 1.
Is there a way to have bordered posts in 2 columns?
Here is the script I use to add border:
.post {
background: #ffffff;
margin:.5em 0 1.5em;
padding:8px 8px 8px;
border:1px solid #000066;
border-bottom:1px solid #000066;
border-width:1px 1px 1px;
}

and here is the original script for post:
.post {
margin: 0 0 45px;
position: relative;

}
I noticed that if I replace the original script, I see again the 2 columns, but not bordered.
As I am a beginner of css, please tell me easily way. Thank you.

Roberto