The best way to do it is to have a rack like the following. Create a separate mixer for the Redrum. Then wire each Redrum channel into a separate mixer channel. Remove the wiring from "Stereo Out" on Redrum. Instead connect the new mixer into the place where the Redrum was previously connected on your main mixer. Now you can control each channel separately.

Hold shift when you create the reverb FX device. That will prevent it from wiring automatically into the wrong spot. Then press TAB to go into the patch cable mode. There drag the cables the 2nd channel of Redrum into the new reverb module "Audio Input". Then from "Audio Ouput" drag the cables into the Redrum mixer channel 2.