Opening 2 aweber accounts at the same time?

I'm not from aweber, but with my personal experience with them, it should not be a problem as long as you don't plan to use them for spamming or other illegal purpose. People may want multiple accounts for different niche list building purpose.
But the best is to check with their support before you open 2 accounts for the safe side.

You got me curious so I did some digging and came up with this:
1. Their terms of service page make no specific mention of multiple accounts. I did find one interesting clause: "System accounts can not be transferred or used by anyone other than the subscriber. Accounts which have been transferred to other parties, or show other activity in violation of this paragraph, are subject to immediate cancellation. "

I don't think that's directly relevant to your situation, however, there's a chance they could misinterpret a second account as a transferred one which would violate terms of use. I'd clear it with them first.

2. Their FAQ page has the question "Do I need to order multiple accounts?" Their response is that is not necessary--they point out that you can setup all the lists you want and will pay for on a single account--but they don't say that it's a violation of terms to sign up twice.

If I were a betting man I'd say you're alright, but why take the chance? Send an email and get the peace of mind.

That's a good question actually. What if you already have an account and it's set to double optin? As we know, there is no switching back, so some one might want to start another account to have single optin.

theres no prob I have had 4 accounts for 3 years