if your young in the game I wouldn't exactly worry about it yet, but you should think of the pros and cons for both, for example, if you have your second polis in the same island you can support it faster, if you don't you can conquer more reach farther places, over all, I would keep my second city with my first polis and build on that island, after that I would leave my comfort zone if I feel confident enough, but it is your choice.