SDL 1.3 multi monitor behaviour

Hi Folks

My app opens 2 windows on one monitor and works just fine. Now I want to be able to open the 2nd window on another monitor, which will always be connected.
I assumed it was as simple as creating the 2nd window with the x position as the start of the 2nd monitor (as my desktop is extended across the 2nd screen).
This clearly isn’t the case as my app crashes somewhere - even if I just have both windows created on my main monitor.

Does anyone have any info? I know SDL can get the details of the 2nd monitor, but do I have to do anything special to display my 2nd window on the 2nd monitor?