Multi select check box has always been one of the missing type of fields with Dynamics CRM. This time around one of my old colleagues has developed a great community supported edition for the Multi select box utility.
It can be downloaded from here:
It has a great manual alongside. I did try it out myself with CRM 2015 Online Update 1 and works great:
Step 1: Import the solution
Step 2: Go to the entity form and add multiple two option fields(each for one select) on a section and Add the following webresource with parameters as the two option names separated by ;
Step 3: Publish the form, our work is done here.
I kept the two options on the form visible for testing, in real scenario you can mark them not to be visible:
Both fields not selected:
Both fields Selected:
One field selected, other not selected:
This is a very good user experience and great work done.
Hope it helps and Happy CRMing!