Safety in what sense? I'm not running a multi-PSU setup now but used to do that when 450W~500W was the highest capacity readily available. Since you're using extenders, I assume you're not putting all 6 into a single case so heat build up shouldn't be an issue.
Safety in the sense that the cards would be drawing power from the motherboard of one PSU and the PCI-E connectors of another PSU. Additionally, I have heard there are issues with overloading the motherboard ATX header. Also, I have read there are issues with having two PSUs wired in parallel that are not grounded together. These would require between 1200 - 1700w, depending on what cards I use. That is probably enough to overload one wall socket, so they would probably have to be grounded at different points. Looking for a little reassurance that someone has done this with high wattage GPUs, it is working fine for them and what setup they are using.
Starting a fire, burning up thousands of dollars worth of gear along with my place is kind of something I would like to avoid...
@bitlane - what cards are you using? Let me know how that works for you. I used the paperclip method and had a 550w as a slave powering one card.