The twin gearbox only really needs one litre of oil, the factory specs say 2 pints (1200 ml's) so fill with one litre, then add another 100ml's.......If the oil is just showing on the end of the dip stick with the bike upright on level ground, then that is enough. The leak is probably from between the output shaft bush and the input shaft, common leak if the box oil level is a tad high.......and/or if the output shaft bush is excessively worn.