The use case I had in mind was more about sharing use of expensive servers than client/desktops. If malicious code can hijack a web browser and break its sandbox in order to grab a shell on either a single or multiuser system then you've got much bigger concerns than whether more than one mutually trusting parties share machine use. Another security layer is needed if this risk needs mitigation for you or other users, e.g. running different instances of the browser within separate VMs or using suitable MAC policy enforcement tools on the browser software. One extra sandboxing layer instance probably wouldn't be enough, given that CSRF and XSS threats between sites regularly visited are going to be a greater risk in practice than a malicious website code getting shell on the client machine. As a minimum I'd recommend all financial browser use should be in a separate VM per user.
The issue here is far more to do with whether the browser is secure and needs fixing or containerising regardless of whether on a single or multiuser machine, than whether Alice and Bob can trust each other not to use such a browser when they have different accounts on the same shared machine on which typically either Alice or Bob but not both installs this software.