Refilling your own tank may be illegal where you live and there are serious potential DANGERS.
Don't actually do this! It's a hypothetical to be used after the end of civilisation following the singularity, or if you are marooned on a desert island and need to do a transfer to a small tank to trek across the island to find help. That being said this is probably safer than types of transfers involving liquid syphoning where the bleeder valve has to be opened which increases the risk of fire or freezing burns. Let me stress again, messing around with HIGHLY FLAMMABLE GAS UNDER HIGH PRESSURES IS VERY DANGEROUS.
A temperature gradient is used to transfer the gas. Two tanks are joined and one is kept cooler than the other. The propane will condense in the cool tank and grandually fill it. This works because the liquid and the gas above it in a tank are in equiblibrium for any given temperature. The equilibriums is shifted in favour of liquid by introducing the cooler tank, where gaseous propane will condense.
1. Join the two tanks with a HIGH PRESSURE hose rated for full propane pressure.
2. Place the tank you're filling in a bucket of iced water, the source tank can sit in the sun/ bucket of luke warm water.
3. Open the cool tank valve, then open the warm tank valve.
4. I havent yet measured the rate of transfer but youre looking at half and hour plus to half fill this 2kg tank.
Keep monitoring how much has been transferred. Overfilling a tank is very dangerous.
5. Turn off the warm tank first so the residual pressure in the hose is minimised.
6. Turn off the cool tank, and remove the hose CAREFULLY. There will be some gas under pressure in it, but no liquid gas if both tanks were kept upright.
A Few Thoughts:
Don't try turning the filler tank upside down. Trying to transfer liquids will increase the dangers hugely. There are other Instructables on liquid transfers, but I also would' t recommend unless you're a pro in this area.
This process is slow and could't be left unattended, and obviously should have no ignition sources near. The worst case scenario I can forsee so far is a hose burst resulting it two cyclinders flowing at full tilt. I think the risk of a hose burst should be no greater than if you were attaching a normal high pressure stove to it, in fact I imagine the pressure in the 2 cylinder system should be roughly that of propane at the temperature of the cool tank. It probably would not to be a good idea to heat the warm tank to more than 40 degrees Celsius, which is the suggested max temperature.