In days, where Axel left for like 15 days with the other Organisation members to CO (can't remember) CoM happens, so Days is kinda interlinked with CoM. So it's days, CoM then days again...
Erm, hope you understood that..
Actually Days and CoM are intertwined...CoM started during Days and also ended during Days. For a short period, both games are running parallel in the timeline.
It's only that CoM displays Sora's and Riku's side at that time, while during the period CoM happens in Days, the focus is on Roxas and Xion and what they did while CoM plays out in the background.