That depends on the company that they work for - most companies strongly discourage moonlighting as it opens them up to liability should your side work fail and you get sued.
In lawsuits, generally, the deep pockets get the most attention so your home company would be a target, even if they had nothing to do with your side design.
Legally, if its OK with your company to do this (which I doubt) in the US, its not outlawed.
Keep in mind, that if you work on the side, you also do not have your companies corporation to shield you from lawsuits. Got any insurance of your own?