2 GPU’s risers / SATA cable may be stable but it is NOT safe long term.
As Sprucemoose said: The SATA connector is rated for max 4.5Amp 54Watts. The PCIe 3 specification allows a GPU to draw 75Watts from the PCI slot. Just 1 GPU can draw too much power from a SATA cable. A well built and safe rig with have NO SATA cables anywhere, except the Hard Drive.
Please read the following post:
The FACTS about GPU mining electrical specifications - #36 by sampson