Thank you so much for helping me with this query.
The documentation surrounding the AD9984A chip I have found to be rather light, when compared to the user manuals of ADV7282A-m or ADV7401 or ADV7482. Thankfully, those other manuals are able to add more detail to what is found in the AD9984A datasheet.
Are there I2C registers available that allows one to choose a output test pattern? e.g. Colour Bar pattern, or Checker Board or Gradient Colour pattern.
At the moment, I think I can force the AD9984A into Free-run mode and use the default "blue" screen to synchronise with the second chip in the conversion phase.
But any advice would be greatly appreciated.