Is it possible to run bitminter on 2 computers with one account? I have 2 Ubuntu 12.04 machines. I'm running bitminter on both. But the website in the workers section only lists what one computer is doing. 
Do I need 2 accounts for this setup or something?
You can use separate worker accounts if you want separate statistics for your machines. Otherwise just run all of them under one worker account.

If some of your workers are slow they may show as zero hashrate at the website. This is because pool websites estimate your hashrate based on how often you send in proofs of work (aka "shares"). For slow workers this does not happen very often and the website is unable to make a good hashrate estimate. As long as you see the worker statistics for accepted proofs of work going up everything should be fine. View the worker stats, wait a while, refresh the page - if the stats went up, it is working.