Can a user licensed with a CAL access multiple instances of Dynamics CRM?

The answer is yes!

As per Microsoft Dynamics CRM 2015 Licensing Guide, "A user or device licensed with a CAL may access any number of instances of Microsoft Dynamics CRM 2015 or earlier versions running on the organization’s servers."

Download the Licensing Guide here: http://download.microsoft.com/download/E/E/A/EEA7C859-6B4F-40B6-9AE7-CF8C07CA64BB/MicrosoftDynamicsCRM2015On-PremisesLicensingandPricingGuide.pdf