Adam and Steve both died last night.
Cause is completely unknown--they were healthy and flitting around yesterday, no odd behavior, no apparent ill health, no foreign substances got into the tank to my knowledge, and the usual things that might take out a fish, like overfeeding or old age, I wouldn't expect to snuff them both simultaneously. Water is clear and clean, nitrogen cycle was fixed in advance...huh. I turned on the light this morning, saw they were both tucked in at the bottom in what I assumed was the usual sleep mode, but this afternoon went to feed them and they hadn't moved. A tap at the tank set one loose and belly up, and the other, fished out, proved to be the same.
Most likely possibility that presents itself is a heater malfunction--it got cold last night, they're near the sliding glass door, and if the heater wasn't up to snuff, it's just possible that the temperature dropped enough locally to send them into shock.
Second possibility is that there's a reason Petsmart was selling their platys for a buck fifty apiece.
I'll check the heater, wait a week and let the tank cycle enough to hopefully clear out any other possible toxins, then try again, I suppose. Poor little guys. I mean, I don't get attached to fish, particularly, they're more ornamental and interesting than lovable, but you still feel a twinge of guilt when they snuff it on your watch.