I know that there are purpose built, slick siphoning kits that you can buy that allow you to transfer fuel bike-to-bike, but here's a cheap & effective alternative for your DIY'ers.
Note: if your fuel tank has a screen in the filler cap hole that is not removable, this technique will not work as the donor bike.
1. Fuel line hose (I prefer clear).
2. A small shop rag.
You'll need to make two lines; one long enough to reach from bike-to-bike, all the way to the bottom of the fuel tank on the donor bike. 4-5' is should be sufficient. The other hose only needs to be about 1.5' long.
How to Use
1. Insert one end of the longer hose into each motorcycle gas tank.
2. Insert the shorter hose into the tank of the donor bike. It does not need to be deep into the tank. A few inches is adequate.
3. Use the small shop towel to create a seal around the two hoses that have been inserted into the donor bike's tank at the tank's filler hole rim.
4. With your hand securing the towel to maintain a seal, blow strongly into the shorter hose. This will create positive pressure into the donor bike's fuel tank, forcing fuel to escape to the recipient motorcycle. You may have to plug the hole with your tongue and blow more than once to get the fuel to being flowing.
Once the fuel beings to flow, it will continue to do so because of the vacuum that you've created. However, gravity is in effect, so the further the recipient tank is below the donor tank the better. To stop the flow, simply remove the long line from contact with the fuel on the donor bike.
This kit is cheap to make, easy to use & store, and best of all, no risk of fuel coming in contact with your mouth.