Bananas are climacteric fruits and so ripen mainly from the action of ethylene gas that is released by the banana itself. The stem of the banana is where most of the gas is released. By keeping your bananas together in the same container the cumulative effect of all of them releasing gas ripens them more quickly. If you want them to ripen more slowly then you can separate them and cover the stems with cling film to quell the release of the gas. This is as much as I know, but but separating the bananas the stem is 'opened' and so more gas could be released, so separating them and still keeping them in the same container will make them ripen more quickly. They will ripen faster only if they are plucked, but then stored with the other bananas. The effect of 'opening' the stem by plucking it from the bunch is going to be counteracted if you remove it from the ethylene rich environment where it is surrounded by the other bananas.
