Active/Standby load balancing dynamically with LTM

I'm not sure if I'm using the right terminology, but some application folks want to set up identical web servers where they can test upgrades and changes to server A while maintaining production traffic to server B.

LTM is currently directing traffic for the application using an iRule and matching URIs so it looks like:

www.company.com/app1 -> App1_Pool -> Node1 & Node2

Is there something I can do to intelligently determine which node (or a different pool) this lands on without administrator intervention? Ideally something that they can trigger themselves when they are ready to upgrade their apps.

www.company.com/app1 -> App1_Pool -> Node 1 (if active) or Node 2 (if active)

I thought of them possibly disabling the site in IIS which would make the port monitor go down and stop traffic flow but they want to still access the app in a testing capacity so that likely won't work.

Any help is appreciated, thanks!