As most of us are aware, each instance of SSAS 2012 may be run in either tabular or multidimensional mode (and not both). This necessitates having 2 instances should you wish to do both.
HOWEVER, I picked up something interesting at the PASS Nordic Rally. There is an UNSUPPORTED 'toggle' that you can change in the MSMDSRV.INI File which will allow you to use the same instance in both modes, albeit one mode at a time. Doing so DOES necessitate either a stopping and restarting of the service and/or restarting the box. The person that put me onto it works for a major US consulting firm and does this all the time.
Here is the link to the necessary documentation from the books online.