Update:
I followed the guide I linked earlier, as closely as I could. Using the "starting values" from OverdriveNTool gave me immediate artifacting and BSOD within a couple minutes, so I went back to stock values on the core/mem clocks, and kept the fan speeds high. I was able to get Cast XMR to do cryptonight-fast (masari coin) at about 3500 h/s, but not stable longer than 5-10 minutes. So I went to xmr-stak and I've had it doing about 3300 h/s and stable for the last 6 hours.
Unfortunately, I could only get this result using one GPU. Any combination I've tried so far of using the second GPU, I get immediate artifacting. So I'm going to enjoy my small piece of victory pie for tonight, let it mine, and tomorrow I'll start experimenting with swapping GPUs into different PCIe lanes, swapping out different risers, etc. If anyone has suggestions, I would love to try them. To be clear, right now my goal isn't to maximize performance of these GPUs. It's simply to OBTAIN performance on both of them. I'd cry tears of joy right now to have them both running at even 75% of what they're capable of. Once I have that, then I can shoot for the stars.
Thanks again guys.
Theagemaway