A co-founder or officer may (and probably does) have a fiduciary duty to his company. This is a much higher responsibility than a normal employee-employer relationship.
Basically when you have a fiduciary duty you need to put the interest of the company ahead of your own, you should not do anything that might be a conflict of interest.
In general the best way to prevent problems (lawsuits) down the road is transparency. If you want to do something on the side ask your company if it is OK. If you get a written resolution from the board of directors stating that it is OK then it is OK.
If on the other hand you go do something without telling the company, the presumption might be that the reason you were keeping the activity secret was because you believed it to be problematic.