You can split the XLR output of the 990 with an external splitter to connect the center channel output to both channels of the amp, but I wonder what you'll gain over just bridging the amp to mono. You have the same amount of power going to the same speaker, and since you aren't doing active biamping (the speaker's internal crossover is still the point of crossover) you don't have the potential benefits afforded by biamping in that manner.