[CSS] 2 Divs on one line

Hm,

After a while of working at this, I still cant find a solution.

I've tried many things, from two left floats to setting up the widths, they will work in IE most of the time, but when I come aroudn to testing in Opera or FF, they collapse.

I want a table-less setup that has a setup like this:

The Main id will be first, setting up the overall layout.
The Heading id will be next, just a explanation of the script at the top of the main div, i.e. Register, Login.
The main id after, just containg the container classes, right below the heading div.
The Container class will be inside the main div, Containg the Post and Post2 classes.
Post and Post2 are the two divs I need right next to eachother, Post containing 'Username' and Post2 containg the text box. But this is where it messes up. I need them seperate divs, opera and FF like to put them in with eachother.

I don't do css that much, but it is validated so far, so its cross-browser, somewhat.

If someone can help me that would be great, I think I got everything I wanted to say.

Jordan