It's allowed [with the caveat there's no guarantee it won't be merged](https://meta.stackexchange.com/questions/148914/officially-sanctioned-second-account?rq=1), provided that you abide by the good faith criteria.

If you attempt to actually run something like the script in your [previous post](http://stackapps.com/questions/6944/autodownvote-se-browser-extension-automatically-down-votes-every-single-post-y), for example, both accounts will likely be suspended without second thought. Having a separate account for testing doesn't shield you from the consequences of any malicious behaviour.