I do it regularly. 7l at home, 3l stays in car. I have a Ripley, so my fill connector is a best fittings female quick coupler. I got a 1/8 bsp reducer for the 300 bar cylinder fitting, put on a male - male 1/8 bsp fitting and then screwed on a best fittings quick coupler so it ends in the male fitting. Dead easy, and virtually the same as filling a rifle, just bigger.
Gets a bit noisy if you forget to shut off one or both cylinders before opening the bleed screw.....