contract Mappulator {
    mapping(address => int256) public BigMap;
    struct mapper { mapping (address => int256) map;}

    function MappingCombiner(mapper map1, mapper map2){
        BigMap = map1.map + map2.map;
    }
}

Internal type is not allowed for public or external functions.
function MappingCombiner(mapper map1, mapper map2){
                         ^---------^

Location has to be memory for publicly visible functions (remove the "storage" keyword).
function MappingCombiner(mapper storage map1, mapper storage map2){
                         ^-----------------^