Problem solved. Very strange, but inserting a brandnew GT8800 in the second slot suddenly gave me a display!!!!! Then I inserted the HD4650 (not 4850 as I mistakenly wrote above) in the second slot, and I have display too!!!!!! Then the first slot with the other HD4650, and I have crossfire!!!!!!
I can't find any logic. AMD/ATI helpdesk baffled too, they wrote me a new driver is neing released shortly preventing this type of problem.