Exodus - Chapter 7

God sends two witnesses to Pharaoh (Moses and Aaron) to warn them of his coming mighty acts of Judgement.  And when the Egyptians do not listen the first plague follows - Aaron raises his staff and strikes the water of the Nile and all the water was changed to blood.

Similarly, we see God send two witnesses in Revelation 11 to warn of the Judgement of the world.  And in verse 6 we see that these two witnesses have power to turn waters into blood