I want to test my Stack App to make sure it works properly without and problems. Is it allowed if I use a separate account just for this purpose?
It's allowed with the caveat there's no guarantee it won't be merged, provided that you abide by the good faith criteria.
If you attempt to actually run something like the script in your previous post, 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.