If you want a list of lists you might also consider using associative arrays.
MainList = associative array();
AList = {"a1", "a2"};
BList = {"b1", "b2"};
mainlist["A"] = alist;
mainlist["B"] = blist;
show(mainlist);
Result:
mainlist = ["A" => {"a1", "a2"}, "B" => {"b1", "b2"}];
Check the documentation for more information about associative arrays - they're extremely powerful.