Back up to 2 GPUs!
Added 2019-07-31 23:55:45 +0000 UTCI finally seem to have a reliable power supply! After the first one exploded, I received a replacement of the same model on Monday (Rosewill Photon 1200). I installed it, but just like the previous one as soon as I started rendering, my computer shut itself off with an "anti-surge" warning (even with just a single card powered). Instead of giving THAT one a chance to explode, I just asked for a refund and went with a different brand entirely.
I went with an EVGA Supernova 1000 G+. So far, it's working perfectly! No surge warnings, and both GPUs are rendering just fine =) I'll be monitoring the temperatures closely over the next couple days though, as that's my only remaining concern. The GPUs do a good job of keeping themselves cool, but they pump a LOT of heat into the rest of the case, which could damage other components.