Wondering if you can use a single power cable plugged into a PSU that splits off into 2x 8 PIN power cables for these GPUs.
 
Example, can I use just one of the PCI-E cables here:
 

 
Each cable splits into two 8 PIN connectors (technically 6+2). PSU is a Seasonic 750 Focus with the 3070, and the other PSU is an EVGA 650 G3 for the 3060ti. 
 
GPUs are:
RTX 3070 XC3 Ultra Gaming
3060ti FTW3 ULTRA GAMING
 
I ask because I see a red LED light flash above the PCI power cables on the GPUs once during power on with both systems. I don't recall seeing it before, but maybe at some point I removed the extra PCI-e cables and just went with the "daisy chain" solution.

Hoggle
It’s ok to use one cable with two leads off of it. Normally it’s best to use two separate cables but one cable is fine especially if you are not overclocking.

The only thing is you don’t want to use third party cable splitters. If the cable came with the PSU you have nothing to worry about as it’s designed to meet the standards.

Thanks. Cables are both the ones that came with the PSUs, Seasonic and EVGA respectively.

Gamefever
I will say, no it's not ok to use 1 cable.
Really depends on power needs for the graphics card and in every case that comes up you have more than 1 connection for a reason.
Each cable has set amount of power that can travel along it.
Splitting that means less power to each connection, not each connection gets x power.

Simple solution is just to plug in the extra cable then. Both PSUs come with extras, one is actually the 3070 PC and connected to the PSU. I have been swapping GPUs back the past few days and the other GPUs use a 8 PIN. I pushed the second cable to the back of the case, probably when putting in the lower power draw GPUs. Makes sense why I never saw it before as I plugged in both cables. Just forgot to pull it back up to the front of the case when I put the 3070 back in.