Is it is legal to work for personal use/in another company, while working in another private firm?

I'm working as an iOS application developer for the past 2 plus years. Now I would like to register my own developer account to iTunes for uploading my private apps.

1. If I create an iOS individual developer account and uploading my apps to the iTunes store, and at the same time I'm working in the previous company, then is it legal?
2. If I create an iOS company developer account ( developer account for companies, after company registration) and uploading my apps to the iTunes store, and at the same time I'm working in the previous company, then is it legal?

Its not illegal.

But most companies have clauses in their employment contract about it.
So a couple of steps you should do.

1. Make sure all Development was done in your time AND on you equipment.
2. Read your contract
3. Talk to them about moonlighting see how they (your employer) feels about it.

   Most companies will be fine as long as its in your time and not competing, but best to check.
   

   • Get something in writing if you can that says that it is fine.


4. If they are adverse to you moonlighting (even if you think it is OK by your contract) they can make your life miserable. So be careful about taking the next steps unless you have their blessing. A lot of states are employment at will so the best that could happen is you loose your job the worst they try and sue you for your product after firing you (can you defend yourself against an aggressive corporation?).
5. Note (4) is worst case. You are probably a minnow and unlikely to warranty any effort. So get permission and things will likely be just fine. Don't ask and start making lots of money then you could end up with issues.

Just to add to what has already been said, I have always written it into my non-compete agreement with the company I was working for. If I started a project on the side just like you're suggesting I will talk to my company HR representative and add the new project to the non-compete agreement.

It'll depend solely on your employment contract with your main employer. Most employers will have a clause that prevents you working for another company in the same space (ie on a product that competes with them), but especially if you just want to publish some apps that you've been working on in your spare time you can always try to negotiate with them - or at least get them to acknowledge that specific apps are outside the non-compete clauses of your contract.