<<... How does it "know" what each flsh is aimed at so that it doesn't
try to increase the 420EX duration in order to provide more light to the bug
and therebye burning out the background or try to better light the
background
by increasing the flash duration from the 550EX and therebye burn out the
bug? >>
Short answer
If they are both set to the same set (A or B), it fire a preflash from both
and sets the flash exposure output accordingling; if one is "A" and the
other is "B" they get their own preflash.
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