There are a few people on cochlear implant forums who have different brands in each ear. There are also partners where one partner is bilateral with one brand and the other partner is bilateral with another brand.
I understand that having a second implant means starting the training process again for that side of the brain, and the results won't necessarily be the same as the first implanted side, so results are down to more factors than brand - hearing history, length of deafness in that ear, amount of rehabilitation training done etc. It's all so complicated!