In order to be able to enter multiple Colorado 529 account contributions, answer the question "Are you or your spouse the account owner?" with No.

This should open another screen to allow you to indicate the account owner name, Social Security number, and your contribution.  Then, after you are done with that entry, you can choose to add another account if needed. 

To go to this area of your Colorado state return, look for the screen titled "Here's the income that Colorado handles differently" and click Start or Update beside 529 CollegeInvest Program Contributions.