At 220w the 3070 technically shouldn't require the 6-pin at all except for overclocking headroom, so a two molex to 6-pin might be OK but it is 40w higher than the 1070Ti.
That is a lot of money to risk though, given the threads on how unplugging a power connector while running killed their GPU. So I would much rather use the 6-pin on the same cable.
Fun story, I burned up a two molex to 6-pin adapter (as in it caught on fire) because it was a counterfeit. The wires were all marked 18ga but after the insulation burned off I measured them at 28ga. Someone went through the trouble of counterfeiting wires with extra thick insulation to save a few pennies. The 250w GTX480 was unscathed.