The tension spring solves one problem but it creates another friction. The additional friction from the tension spring is responsible for the link drop in balance amplitude you get when you switch a chronograph on which can be up to 30º. This is link acceptable in a properly adjusted watch but as you can imagine if link you have two chronographs going at once and both are controlled by a single balance you get an amplitude drop of up to 60º which is unacceptable. link