In earlier version for NAV 2013 and NAV 2013 R2 we have to do some registry settings to run side-by-side on same workstation.
Yesterday I have install NAV 2015 on my laptop which have already installed NAV 2013 R2 version. The Installation of NAV 2015 was completed without any issue. Then I start checking some basic tests like Role Tailored Client of NAV 2013 R2 and NAV 2015 working fine together, if you run any page or report from development environment of NAV 2013 R2 then it will automatically opens in NAV 2013 R2 RTC without any client/server version issue. The same scenario working fine in NAV 2015 also.
Second I have test administration management console of NAV 2013 R2 and NAV 2015 both working fine together. You can manage any version service from respective administration console.
So as of now as per my experience NAV 2013 R2 and NAV 2015 working fine side-by-side. I am still going through some more test cases if there are any issue found then I will post here. But I hope that Microsoft takes care of all these scenarios and not any major problem we have to face.
Good Work in NAV 2015… Thanks to Microsoft…!!!