Our team evaluates Clockwork + ActivityTimeline as a replacement for Everhour plugin. The key feature that Everhour gave us is clicking "Start Timer" in the given task, that causes disabling of any other timer that counts time. It would be great to have the same behavior in Clockwork. The key advantage of Clockwork is integration with JIRA time tracking on which ActivityTimeline rely.
To be honest I don't see a use for multiple timers working at the same time. If it is really needed it should be optional.
Do you consider implementing that feature?