I think a crossover would be simpler if there were more of a cameo situation than trying to combine settings, but fans of one or the other would probably be disappointed.
I came up with two silly examples. They turned out much more detailed than I anticipated.
Pokemon version:
Ash and co. run into Kari, who is being chased by Archnemon and Mummymon in human form. The two villains are new hires at Team Rocket. To steal pokeballs, Archnemon uses her KoDokugumon and threads while Mummymon uses bandages and zombies. However they send random Pokemon to battle instead of attacking anyone themselves. Ash drives them away and Kari takes him and his friends back to Tai who explains that the villains used BlackWarGreymon to get to the Pokemon dimension. Ash and co. agree to help stop the villains, while Jesse and James join them in the background because they are extremely jealous. Meowth comically falls in love with Tailmon and she scratches him in the face. Ash and Tai challenge the villains while their friends stand around and comment. As the protagonists are about to win, BlackWarGreymon interrupts. Mewtwo shows up from far away just in time to stop him. The two battle for a few seconds until Ash convinces them to put aside their differences. Bewear comes in to beat up Archnemon and Mummymon when they try to flee. Jesse, James, and Meowth try to sneak off with the stolen Pokeballs, but Agumon and Pikachu blast them off. The protagonists laugh, xylophone plays, and the camera pans to a sunset.
Digimon version:
During a battle, a popular Pokemon trainer is suddenly corrupted, forms a mask and cape, and laughs maniacally. Meanwhile, Tai and the other chosen children are going about their daily lives until they are sucked into a portal. They and Ash's group are brought together somewhere by an entity called DigiPokey, which isn't explained any further. Masked man arrives to steal the Griseous Orb. Pikachu and Greymon, along with the other (un-evolved) Digimon and a few other Pokemon are sent out, but they are overwhelmed by things like noiverns and Devidramon. Tai is angry so Ash gives him a pep talk and then they go after the villain. They find the masked man was being used by a Digimon to summon Giratina as a way to escape from another dimension. Vamdemon again! He fuses with Giratina into NeoVamdemon Darkness Mode. All of the chosen children's Digimon evolve to mega and Ash, Misty, and Brock mega evolve Charizard, Gyarados, and Steelix. Each of those gets to attack exactly once before being smacked away and devolved. Pikachu and Agumon fuse into Omegamon Gigavolt Mode, which has gold armor, rosy cheeks, and pikachu parts for arms. The villain is defeated in one slash and Tai and Ash clap hands. In the background, Matt sulks and Gabumon tries to cheer him up. The protagonists wave to each other sadly as they return to their respective dimensions.