Word for two or more feasible yet opposing explanations for the same observations?
Solution 1:
I think the expression competing explanations fits the bill here. It's a common-enough way of expressing the idea that there are multiple explanations for some phenomenon, each (at least as far as our state of knowledge at the time of expression goes) with some merit and some likelihood of being correct.
In OP's sentence we might replace the underscores thiswise:
My my, Dr. Geometrigonomestein, we have here a competition, for both explanations are quite complete and believable indeed.