Only use splitters if you know exactly what you're doing.
What he/she said. Are you using the 6+2 pins for the risers? Use SATA converters for those, you can even double them up if needed, but don't skimp on the cards themselves. Or pick up a cheap 400W psu for those two cards, they're not very expensive.
Also: the 1070 will draw more than 100W, up to 150W actually. You can keep it low by underclocking or mining a cryptonight coin.