I generally speaking wouldn't try to merge two asyncronous signals and multiplex them onto one line with the idea of making two DMX-512 streams co-mingle into ONE single coherent stream.
(This is as opposed to say muxing multiple terminals or modems, then running that down a T1 then reverse muxing it out at the other end, in which case all async streams are kept sepparate despite travelling down one consolidated cirucit)
(Hell, it ain't even the same as using a CSU to break off voice or data channels off a T1 and direct those into a PBX, since that would most likely be a partially channelized circuit, or at least the voice channels would be).
But they do make DMX mergers, so there. I wouldn't use it. I can see the need, but I see a bigger need for the proper equipment from an operations perspective first, which would eliminate the need for a merger in the first place.
Regardless, sure, take that into an Opto Branch 4 and send duplicate signals in up to 5 different directions(they say 4, you can do 5, in face you can do 10 if you use DMX 3 to 5 wire converters as necessary).
Based on descriptions, the DM-2512R would be the merge device, and then you'd patch that to the OptoBranch 4 and then you can split that up to 10 directions(using adaptors as needed).
I suppose short of getting another console to do the tasks of those two lesser consoles, this could work fine. I personally wouldn't do what you're doing, but it would appear you've selected the right gear and it should work fine.