The first mind merge devices were created by the Lights. They were used to merge Light minds with Core, the artificial mind they created to predict what they were intended to do as a species. The Lights device left the original mind intact, connecting various components directly to Core. The end result was Core becoming a larger more powerful intelligence, but with multiple personalities operating at the same time.
When Eximius had her visions of merging a Light mind with a nexus mind, the Dren decided to do it in a slightly different way. Because Core sometime had difficulty making decisions due to conflicting personalities, they would only merge two minds with similar views. They would also not leave the mind fully intact. Instead each component of the minds would be used as a building block to create a completely new mind. All memory and personality components would be kept, but redundant pieces that connected everything would be discarded for later study.
Unlike the Lights, who have never fully understood how minds work, mainly because they fear tampering with their inner workings, the Dren have enough knowledge to make complex changes to the construction of any mind. The Lights had feared not only losing the memories of a mind, but also their personality if they examine the inner workings of a mind. Eximius never had this fear because of the visions she had seen of her future. her visions showed her that memory and personality are duplicated across the components of a mind. With that knowledge she designed devices to examine a mind from inside then volunteered to be the first.
The Dren believe that Core will at some point be unable to function due to the internal conflict. Many Dren scientists want to help the Lights fix the problem, but with the Lights fearing them none believe they would be trusted if they offered help. Eximius has hinted at knowing what the fate of Core would be, but never told any details.