I wonder if ValueCount( FilterValues( ... ; ... ) ) would work. There may be custom functions that may help.
Same thought as beverly (which is never too wrong I believe):
list1 = "apple¶orange¶banana¶mango¶peach¶plum"; // list 1
list2 = "cherry¶banana¶melon¶mango"; //list 2
list3 = FilterValues ( list1; list2)
ValueCount ( list3 )