Digimon Story: Cyber Sleuth - Hacker's Memory

You'll probably get this naturally but as far as I'm aware you need at least 2 Digimon on your team to perform a Xros Attack, which depends on the CAM of your team. The more times a Digimon fights in battle the higher its CAM will get until it reaches 100% (this is a long process!) and the higher the CAM the better the chance of a Xros Attack happening.,
My team was a Tentomon (my starter) and a Yokomon (which I Digivolved from Pabumon) and after their first Xros Attack ever it unlocked.