Besides the legal challenges, there are the IP challenges, and if you're honest with yourself you probably aren't doing your best work on one of the two jobs (and likely the one paying the bills) and so you are putting your reputation as an employee at risk as well.
The simple answer is, coming up with ideas? Great to do while you're working. Always be on the lookout for the next big thing. Due diligence on what has been done so far? Also fair game. Developing an itemized list of things you'd have to have done before you were 'up and running', also a reasonable thing. But once you pull the trigger and you're "starting a company", my experience suggests you will be much more successful if you are doing that 'full time' rather than 'nights and weekends.' If only to minimize the cognitive load of things not related to your new company.
Also if you treat your current employer well, which is to say you leave when you get serious about this new company. If it goes to hell they might take you back, if it goes well they might invest, and if you're just looking for mentors you may find them there. If you treat your employer poorly you will not have a chance at those benefits.