You need to add some attribute to your area tags and add it to the default function options, for example:
<area shape="poly" coords="88,265,74,266,65,257,57,226,64,199,75,182,87,181,102,198,106,214,100,240,97,251,86,261,80,263,85,262" href="#" alt="muscle1" group="thigh">
<area shape="poly" coords="123,269,108,237,108,208,114,196,126,183,139,181,145,194,152,218,154,233,152,254,146,272,139,279,120,275,121,267" href="#" alt="muscle2" group="thigh">
JS:
$.fn.maphilight.defaults = {
...
groupBy: 'group',
....
Attribute group="thigh"
is the same for them so they both will be highlighed on hover.
See fiddle