* You need to be extremely careful with IP and legal: are you going to use your work machine for a side project? Are you going to do any side work from your current company offices? Is there conflict of interest? All those things need to be carefully planned.
* You're going to get distracted, so your performance on both jobs will be lower.
* If your management is going to figure it out, you'll lose trust. If I'd figure that one of my employees does contracting on the side, I wouldn't be happy. If you really need more money for a serious reason, I'd prefer people to chat with me about that during 1:1. We've had an employee couple years ago who had a brain tumor and we covered his medical expenses on the company, with no obligations, which was more than his annual salary. He recovered successfully since.
* Having two jobs will impact on your quality of life and your family. Maybe not instantly but at some point it will. Be careful not to burn out.