Systems Management

there's one question here: how are these getting set up?  You'll need some way to do a dual-output.  Do you have dual output or second video cards as standard?  Or are people bringing in their own video cards as well?

More than a technical problem, what you have here is a policy/process problem.  Keeping old monitors should be handled as part of the hardware refresh: you have one monitor, you get one monitor.  No, you can't keep it.  Have your boss buy one if you need a second one.  Even better than a second monitor is a large monitor.  It's easier to use and easier to read.

Then, decide on a standard monitor.  Managing and tracking monitors, which really are disposable, is a pain.  If the department in question gets billed for hardware, them the solution is pretty simple: they pay for this.  Make a manager pay for an employee's personal hardware once or twice, and the problem will come to a screeching halt.  And if that managerwants to blow their budget on personal hardware, that's their problem.  Non-standard monitor?  Sorry, no service on non-standard equipment.  Then you handle the policy/standard problem with policy/standards, and not trying to make technology handle a non-technical problem.