GoForSmoke:
For a light source not too far away.. 2 calibrated detectors with 1 closer to the source and the other behind and to the side may give enough difference in reads to estimate distance to the source, but the distance between front and back needs to be an appreciable fraction of the front to source distance if you want any real accuracy.
I imagine there are all sorts of practical reasons this wouldn't work all that well, but that's a clever insight. I fear you may have ruined my weekend.
"I learn a bit every time I visit the forum.", indeed.