Theres no reason you should want to do that. Are you running at least 1 stereo effect to constitute a stereo sound? Like in a rack effect or nice pedal? A noise supressor doesnt count as an effect. I didnt even know they were stereo and the only reason why I could think of them being stereo is if you already have your signal running in stereo and you want to hush both stereo signals seperately.
You cant split a mono signal and get a stereo signal. Well you can but thats not the right way to do it and unless your running an effect that seperates the left and right sides and processes them differently I dont know why you want to go stereo.
Ill explain the way its done and why your preamp is set up like it is. Your mono signal leaves the preamp and goes to, say, a rack effect. in the rack effect you have the option of using different stereo effects like delay and chorus which process the left and right sides differently. You get 2 outputs from the unit for each side. Now if you wanted to use a hush/noise unit, this is were it commonly comes in and why it has 2 channels. To independantly hush the left and right sides. Then you take the L and R side to the 2 returns in your loop.
Otherwise just use 1 channel of your hush. Im sure it says mono on both left inputs or both right inputs.