How can I use a button in one window to enable a button in a different window?

You can enable/disable the components via a Client tag. Just change the client tag state using one button and it will affect the other. Just bind that client tag to the Enabled property of the component(s)you wish to disable/enable.

If the other window is open, you can do this directly without needing client tags. You can get a reference to the other window using You can then get a reference to any control in the window using something like depending on where the control is in the hierarchy.