Optiplex 3070 SFF. Which SATA cable 3.0?

Hi, yes You can use a power splitter and use HDD and SSD simultaneously.

The blue cable is for sure Sata 3, the orange is used for the DVD, but I don't think it would be worse in speed than the blue cable.

However the port on the board which is not blue is only SATA 2. I would use this for the HDD.

But of course best would be to use an M.2 SSD, that's the fastest. 

Re:  use sata power splitter, can i install HDD & sata SSD simultaneously?

3070 SFF motherboard has only two sata ports.  Does it mean both are sata 3.0?
(i.e., there is no sata 2.0 on SFF)
spec: Serial ATA (2 ports Support Standard Rev 3.0, the rest of ports Support Standard Rev 2.0)

regarding power splitter, you may also use optical drive caddy to 2.5 adapter to install the sata ssd in place of dvd drive.  SFF uses laptop style thin odd.

Then DELL does not list it correctly on page 13 in this manual:

https://dl.dell.com/topicspdf/optiplex-3070-desktop_owners-manual_en-us.pdf

Table 7. Storage
Tower Small form factor Micro
es Supported 3.5”/2.5” (maximum) 1/2 1/1 0/1
Interface:
SATA 2.0 1 1 0
SATA 3.0 2 1 1