That had occured to me earlier when I was guessing at overexposed shots - what I am really hazy about (think on I am still a beginner) is that if the strobe could fire and quench then would it not be fully TTL compatible and consequently the YS90DX would in effect have the same capabilities as the DS125? (or am I bordering on extreme confusion).
I suppose the answer is to set the YS90DX to min light output (-7) when using it as the secondary fill in flash.
As a total aside to this issue I thought you may be interested in disparity of costs between buying equipment in the states and outside - my quotes for the DS125 in the Olympus 3973.65 package have been:
States (Berger) $750
UK $1507
Thailand (dealer #1) - $1320
Thailand (dealer #2) - $1050
Thailand (local dive shop #1) - $1420
Thailand (local dive shop # 2) - $955
Singapore - cannot supply
Quite a spread.