Hmm, it doesn't really make sense for Adobe to offer two versions, but oh well.
You've always been able to keep older versions installed if you want to—I have CS3 (ID 6), CS6 (ID 8), CC (ID 9), and CC2014 (ID 10) all installed on a test drive. As Bob notes it's up to you to decide which version(s) to keep installed. I would assume InDesign 11 will be part of CC 2015.
