And I will grant authority to my two witnesses,
This has a legal sound to it. In fact, it was required to have two witnesses to accuse someone (Deuteronomy 19:15; Matthew 18:16; and 2 Corinthians 13:1)
These two witnesses are accusing the beast (chapters 13-19) of apostacy and desecration of the temple, earth, and human soul. That this verse personalizes the two witnesses with "MY" two witnesses indicates that this authority comes straight from the throne. God always has had witnesses, whether human or His creation (Acts 14:16-17).
and they will prophesy for twelve hundred sixty days,
Note the precise fixed time = 3 1/2 years or 1/2 of 7 years.
clothed in sackcloth.
Sackcloth shows mourning, humility, affliction of the soul, and true repentance. Jews fully understand what this means (Genesis 37:33-34; 1 Kings 20:31-32; and Jeremiah 4:7-8). It is an Old Testament example to the nation that things are WRONG. This symbolism may not be well understood by non-Jews or non-Arabs. Gentiles just don't do this.