James Jordan points out that the two witnesses correspond to two cities and to two forms of judgment (Revelation 11). One weapon of the witnesses is fire, and this corresponds to Sodom, destroyed by fire (vv. 5, 8). Another weapon is turning water to blood and bringing plagues, and this corresponds to Egypt (vv. 6, 8). The first is an Elijah judgment, the second a Moses judgment; the first is royal, the second priestly.
Sodom and Gomorrah are destroyed by fire, but the associated cities are spared, though given a dramatic warning. The blood judgment is more complete: Egypt is completely devastated by the plagues and the exodus.
These two clusters of ideas correspond to the larger sequence of Revelation. The trumpets generally bring fire judgments that are partial (8-15). The bowls of wrath-blood bring a final end to the city (chs. 16-18).
Change key and the pattern emerges elsewhere: Trumpet is witness in word, blood is witness in death. Witness in word is crucial. It is essential preparation for epochal transformations. But the fire-witness doesn’t shake the city of man to its roots. Only martyrdom, witness in blood, does that.