I definitely do not do that. You are literally betraying your current employers. FT means they own your 8 hours a day and you are not allowed to do anything except work for them. You are lucky your FT job is chill but most of the employers whom I know strictly do not allow it. If you work on contract job after hours then there won't be any problem as long as there is no conflict of interests.

I also do not do that from family and personal perspective. I do not live for weekends only. Remember the quote "if you celebrate Friday too much, it might be a sign you need to change what you are doing Monday through Thursday". Before 9AM and after 5PM it is my family time and I do not compromise for anyone. No amount of money will sufficient for you and that is the fact. Unless you have huge debt I would stay away from that habit. Even though your job is chill but you still have to sit in front of computer during that specified amount of time.

Most of the people whom I know who retired regret spending not enough time with loved ones. They never regretted "oh, I could have make 50% more money if I work for this company or do double job". This is just 2 cents but ultimately it is your life and your choices.