A couple good rules of thumb:
The fewer connectors the better. Every splitter/extension/etc. adds another point of contact which means more chance of extra resistance and/or arcing and therefore more chance of fire/excess heat
Never use SATA power for anything other than SATA disk drives. Use molex or PCI-E 6-pin to power all of your risers. Stick to one molex chain for every two risers.
Now as far as using splitters to split 6-pins into 8-pins, and 8-pins into dual 8-pins, etc, you should keep a few things in mind. Every 6-pin is good for up to 75W and every 8-pin is good for up to 150W. So let's say you have two cards that each draw 125W. Let's say they are getting ~50W out of the powered riser. In that case, each card needs an extra 75W. You could split one 8-pin into two 6-pins and rest easy. If you split a 6-pin into two 6-pins then you could be loading the cable 2x over spec. This should be no problem for the PSU itself as long as you have enough amperage available on the 12V rail. You probably have a single 12V rail anyway, so as far as the PSU is concerned, it's not working any harder. The difference is that you are going to have double the amperage running through the wire between the PSU and the video card. Now a wire's amperage rating depends on a lot of factors including diameter, length, and how tightly packed/how much air flow it is getting. If you have short, individually-sleeved wires running in front of your intake fans you are probably fine. If you have long, lower-gauge wires with extensions, all zip-tied into tight bundles packed up against other current-carrying wires or bundled into mesh sleeves you are going to have higher resistance and bad airflow - so that's bad. Basically, as long as the wires stay cool enough, you're fine.
Very useful! How about splitting one 8pin into an 8pin for one card, and a 6pin (via existing split on the cable) connected to an 8pin adapter for the second card. That's still just splitting an 8pin right? Only the second card's power travels over a 6pin line for a short distance. Each card's riser would be powered with its own molex line (or two per molex max).
Where did you find watts per 6pin and 8pin? It seems to vary from PSU to PSU (older ones mainly). I wanted a clear guide on per cable wattage and couldn't find it. Thanks for the tips! My wires are all cold or slightly warm at full OC for 24 hours, but I'm going to change anyways to be safe. Good idea to test 7th first.
