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I dont think that they will cancel each other out, however it is possible that the stronger and quicker developing one of the two(whichever that may turn out to be, if any) may inhibit the development of the other, although probably not altogether IMO.
99 has great upper level presentation but nothing at the sfc. 90 has a good sfc presentation but is fighting the upper levels. 90 will move to the west and away from 99. The upper level low will seperate the two and keep one from killing the other. 90 is in no way a gonner...it hasn't even begun. Its chances of development start tomorrow and Saturday when it gets away from Florida and into some more favorable upper air.