Note: this theory for the Pokemon/Digimon cross over is dependent on my pet theory about the Digitalworld. Meaning, my idea of what the Digitalworld is the bases for the premiss of this attempted cross over plot.

Well, the Digitalworld, at least my favorite theory, is an alternate dimension where the universe is made up of code, instead of matter. When the potential space of the internet formed, it created a demi-dimension that bridged our world and the Digitalworld. Because the Digitalworld is built from data, the code that leaked threw from our world shaped theirs. But only the data that had enough energy to travel thew the bride. Things such as dreams displayed on the world wide web are the only data that posses enough of a 'spirit' to cross over. I.E, this explains why certain things on the net are not represented in the Digital world.
If that idea is true, then a cross over from their would be as easy as the Pokemon world bridging to the Digital world by its own internet demi-dimention.