Why is it when I use the mix bus feature to tie two mixers together, my noise level increases from -50 to -30? I thought by using this feature there would be no adverse affect.
The Mix Bus is passive. When connected via the Mix Bus, the audio level of each mixer falls by 6 dB, so each mixer's program level is now 6 dB closer to the noise floor, i.e., the S/N ratio has worsened by 6 dB.
Your message indicates the S/N ratio worsens by 20 dB, not 6 dB. This is not correct. How have you made these measurements?
Try your measurements powering each mixer via three nine volt batteries. Are the results the same? If not, you may have a ground loop problem between the linked mixers.