Presuming there's no biochemical interaction (there's going to be), basic probability would be to multiply 1/1000 by 1 / 1000; 1x1 =1; 1000 x 1000 = 1000000; odds = 1 / 1,000,000.
However, even those things do happen (purple loot!). Which is why I stressed that stupidity is not the issue. Anyone can get into the situation even if they're careful (unless they're abstinent). The issue is (without the vitriol) how humane the situation is dealt with.
Like, actually humane or PETA humane? (Okay, there's some vitriol)