These two businessmen treat each other like brothers and have a very close relationship.
Tips
usage
称兄道弟 can be neutral (genuinely close friends) or slightly ironic (implying people who are suspiciously buddy-buddy, possibly for self-interest). Context determines the tone.
memory
称 = to address, 兄 = elder brother, 道 = to speak of, 弟 = younger brother. They call each other 'brother' both ways, older and younger, showing total equality.
