Ancillary question, it is safe to Y-split an 8-pin to power both an 8-pin 1070 Ti and its 6-pin powered riser?
Generally yes, as the PCI-E spec rates an 8-pin power connector for 150 watts and the wiring for such a connector is normally capable of handling a little more than that safely - *IF* the 1070 in question is something like the EVGA SC 1070 or the Gigabyte ITX that is only rated for 150 watts.
If you are running the cards at a "lower than spec" TDP where the power you are SETTING the card to is 150 watts or less, you should also be OK - though sometimes when Windows crashes it will cause Afterburner to lose it's settings on one or more card and allows the card to reset itself to FULL TDP.
180 watt cards should be OK - most PCI-E connectors have wiring that will handle that safely even on "single connector" cables, and any DOUBLE connector cable (the kind that have both an 8 pin (or 6+2) AND a 6-pin on the same cable) should be safe to at least 225 watts.
That MSI Gaming monster card with the dual connector - might be a bit marginal but SHOULD be ok, it might be rated for 240 watts but pushing a 1070 hard enough to actually DRAW that much power for more than a short peak timeframe takes a ton of work.
Technically, the actual CONNECTOR is rated for 288 watts, but it's rare that the wiring FEEDING the connector is rated for quite that much.
In most cases, you can run 2 risers from a single Molex power string - but check it after the rig has been on for 10 minutes or so and make sure the wires aren't running hot. Not all power supply makers use the same gauge wiring for their "peripheral" connectors, and while the CONNNECTORS are rated for a bit over 150 watts it's rare for any device connected to them to draw more than 50 or so (and most hard drives draw less than 1 AMP or under 12 watts from the 12VDC line, though some high-end FANS can draw close to 4 amps at 12VDC).
