Does ripening banans in a bag with other fruit harm the other fruit?
It doesn't harm the apple, but it does speed up its ripening as well. And that risks over-ripening. So your perfectly crispy apples may begin to become mealy. Eventually they become targets for yeasts, molds, etc and start to rot. But it's not making them toxic or causing untoward chemical changes. They're all just speeding each other up.

Yes, you can just put them in a bag without the apple. They will produce their own ethylene (the gas that causes ripening, and which they make while ripening). It may take longer, since under-ripe bananas will put out less ethylene than a ripe apple, but just by a day or two. It's just a question of how hungry you are.