Nirvana On The Fly (a subsidiary of DRAGONtail Tenkara) has some spectacular line management options, including their Nirvana Line Holders. These are available in three different thicknesses, regular, medium-thin, and extra-thin. These line holders are as good or better than any of the Japanese options available.
But for some, a double or dual line spool is desired. So, in answering that wish, Nirvana On The Fly will soon release a dual line spool. These are the same high quality and excellent design as their single line spools, but with the ability to carry more than one line.