In the past, for a period of time, I took one full time job as company employee and another full time contract job at the same time since both can work at home. I did not let either of employers know the double job. Will this affect my security clearance?
Sounds like you were double billing for your time? Did wither company have rules about doing other work while on their time? Or requiring notification if you sought other work? Propriety information is usually a concern. No telling based on the information you have provided, but there is a good chance you did something under the table and it could be viewed as derogative.
From the employer perspective, he do not want its employee doing other work. But my concern is whether clearance investigator care about this kind of thing? do you think investigators only want to know my performance, personality, etc. when they doing interview with my former company?
Investigations are done for the purpose of determining whether an individual is honest, trustworthy, reliable, complies with rules, regulations, and laws, and is morally sound. Misconduct in employment is an issue.