$http.get(BasePath + '/api/documentapi/GetDocuments/' ,
                                {
                                    params: {
                                        PrimaryID: ID1,
                                        AlternateID: ID2,
                                    }
                                }).then( ...

public DocumentsDto[] GetDocuments(decimal? PrimaryID, decimal? AlternateID)
    { ...

http://BaseServerPath/api/documentapi/GetDocuments/?PrimaryID=1688

http://BaseServerPath/api/documentapi/GetDocuments/?SecondaryID=154

http://BaseServerPath/api/documentapi/GetDocuments/?PrimaryID=1688&SecondaryID=null

public string GetFindBooks(string author="", string title="", string isbn="", string  somethingelse="", DateTime? date= null)
{
    // ...
}

public DocumentsDto[] GetDocuments(decimal? PrimaryID = null, decimal? AlternateID = null)
{ ...

$http.get(BasePath + '/api/documentapi/GetDocuments/' ,
                            {
                                params: {
                                    PrimaryID: ID1!=undefined?ID1:0,
                                    AlternateID: ID2!=undefined?ID2:0,
                                }
                            }).then( ...