Silverlight - multiple ui threads
One UI thread is not enough. There have been multiple instances where during a long-running, non-interruptible UI operation (like loading a child window for the first time) we cannot show any animation (eg: intermediate progress bar).
Having two UI threads would introduce synchronization problems for UI elements.
A better solution is, when showing your child window, do less. Windows should not take a long time to load, so do less when showing that window. Delay processing until idle. Show fewer UI elements initially.