I think with the correct high pressure fittings you could connect them, gas dynamics are such that the pressure between the two would equalize, but I don't think you could get a full 5lbs into the tank and with each progressive fill the amount going into the 5 lber would decrease: Hypothetically, no idea if it will work, if you have a full 20 lb tank and connect a 5 lb tank without additional pressure they should equalize out with about 16 lbs in the 20 lb tank and 4 in the 5 lb tank, then next time it will be 3 and 13 and so on as the pressure is equalizing between the two, and you are not filling the tanks.
Just be aware there has to be a lot of pressure going from one tank to the other to fill them and there may be restrictors on the valves preventing you from doing this, also whatever you use to connect the two canisters needs to be able to hold a LOT of pressure, and don't think normal pipe fittings will do it, they probably won't. but I wish you the best of luck in your endevor, let me know how it works out, as I have contemplated doing this several times.