This is the item in question on Amazon: JBingGG 6 pin PCIe to 2 x PCIe 8 (6+2) pin Motherboard Graphics Video Card PCI-e VGA Splitter Hub Power Extension Cable ( 2 Pack 8 Inches ) ( 6 pin to dual 8(6+2) pin )
 
Can I use this to power 2 6-pin risers from one PCIe cable?
 
Can I use this to power 2 x 1070 ti with single 8 pin power? If safe, is there a TDP I should stay under?
 
 EDIT: didn't realize I couldn't post links, that's the name of the product on Amazon. 

Thanks for answer, but I had to update my post, didn't realize I couldn't use a URL. 

JBingGG 6 pin PCIe to 2 x PCIe 8 (6+2) pin Motherboard Graphics Video Card PCI-e VGA Splitter Hub Power Extension Cable ( 2 Pack 8 Inches ) ( 6 pin to dual 8(6+2) pin )

I myself would Not Use this Single 6-Pin to Duel 8-Pin Splitter
Maybe a Single 8-Pin to Duel 6-Pin Splitter, Maybe.

COMeap (3-Pack) 8 Pin (6+2) Male PCI Express to 2x Molex Power Cable 9-inch(23cm)

I myself would Not Use this in one of my Computers as well.
 
COMeap Seems to make a lot of Cables for Miners.

It is up to you if you want to use them, I am only saying that I would not.
If I had to use one I would at lest use the Single 8-Pin to Duel 8-Pin or Single 8-Pin to Duel 6-Pin
I would never go down a level us a Single 6 to a Duel 8.
You most likely would find a better answer under the Cryptocurrency Area of Forum than under the EVGA Power Supplies Area
 

Yeah, single 6-pin to dual 8-pin sounds really dumb to me.

A PCI-E 6-pin connector is rated for 75 watts. A PCI-E 8-pin connector is rated for 150 watts. Some people say that a 6-pin and 8-pin are basically the same if that 6-pin is using high quality connectors and good, thick wires. Either way, assuming good, quality connectors and good, thick wires, we are still talking about a 75 watt rated connector providing 150 watts to a 300 watt rated load. In other words, you are doubling or quadrupling the safe level by powering two 8-pin connectors with one 6-pin connector. Don't do it.