I checked around, and Spinda has exactly 2^32 variations, or 4,294,967,296. You have a 1/8192 chance of having a shiny Pokemon, so the probability of having just two is:
P(A)P(B)
1/2^321/8192=3.518 E13=chance of 1
so the chance of having two is
P(A)P(B)
3.518E131/8192=2.88 E17
-assuming my math is correct, you have a 1 in 288230376200000000 chance. Good luck with that.
It's about a .0000000000000001% chance by the way.
As of Gen 6 to the present, the shiny odds have been increased to 1/4096. Because of this, the Shiny odds are now .000000000000000002%