Do you know for a fact that your tanks are siphon tanks? If so, they wouldn't be any good for brewing, as the liquid would destroy your regulators.
There are Co2 fillers that you can buy, they basically have two CGA-320 fittings, one that has a valve, and the other usually attached to a high pressure hose.
With typical non-siphon tank, you would need to invert your larger tank to push liquid CO2 into the smaller tank, so having an upside down tank has its own challenges and safety issues.
The other issue that is unlikely to happen is an overfill, but you can weigh the tanks before and after filling to ensure that you haven't overfilled your tank. The reason why this is unlikely, is usually you need to freeze the receiving tank to get close to a full fill, and if you freeze it, you can also overfill. Overfill situations are really just scary when the burst disc breaks and releases all the CO2 at once into your room. If you weigh the tank before the fill (essentially double checking the tare weight), and you know the receiving tank is empty, when you weigh the tank again after filling, you will know exactly how much CO2 is in the tank.
Now, this is all assuming all your tanks are within their hydrotest date. If they are not, you really should bring them to get them tested.
If you don't chill your receiving tank, you will likely not get a full fill due to the equilibrium reached when the receiving tank starts to pressurize.
As always with pressures this high, be careful, do a lot more research, and if you then don't feel comfortable, bring it to the professionals to fill. Saving a few bucks is not worth getting hurt!