Here is the plan for you so that you can accomplish the transfer from Corney to Sanke:
First a picture of the Sanke tap:
The tap has 2 valves in it to prevent back flow of gas and liquid. You must temporarily remove them as you will be reversing the normal flow of both the co² and the beer. Item 1 & 2 must be removed as well as item 10. Once this is done, you can easily clean and sanitize the Sanke keg by turning it upside down and forcing water/PWB and sanitizer into the keg through the beer out connection (The vertical stem) and letting it vent/drain through the gas out connection. By doing it in this way, you will be forcing the liquid up through the dip tube and it will spray across the bottom of the keg (now at the top because it is inverted) and will also wash the sides of the keg interior in the process.
Once the keg is cleaned, sanitized and drained, turn it back over and purge the keg with co².
Now hook your beer out from the corney to the beer out on the Sanke. Force the beer into the Sanke using co² pressure on the corney keg. You can restrict the flow by controlling the gas venting from the Sanke keg.
Once the transfer is completed, reinstall the 3 pieces that you removed from the Sanke tap so that it is returned to its normal operation. Do not forget to pressurize the keg so that you will maintain the proper carbonation level.
I hope this is of some help to you.