The multiSelectRegion param isn't documented anywere (http://jqvmap.com/ or https://github.com/manifestinteractive/jqvmap/), and only shows up in one demo.
There isn't a clean way to access selected regions - just to check the value of the global var "selectedRegions". I think a method should return this (and it should be documented).
I would recommend to switch back to jVectorMap (the project from which jqvmap was derived), since jqvmap seams to be abandoned by its author. jVectorMap supports regions multiselect. Selected regions could be retrieved with the getSelectionRegions method