Can you perhaps also share your system specs with me? Revit relies more on the CPU, while Enscape mainly relies on the GPU - perhaps a dedicated CPU upgrade would make more sense, but please let me know what machine you have and I'll take a look. 
Rule of thumb is that it isn't possible to "split" up GPU usage if you have multiple GPU's. If you have multiple GPU's, the drivers + programs you run will figure out a way to use both at the same time as if it were "one" GPU.
However, since GPU power has greatly increased in recent years, support for this has honestly become deprecated except in specific enterprise+render farm workflows. It honestly causes more problems now than just running a single, but very powerful, GPU. Most programs don't know how to properly take advantage of multi-GPU as such.
If you're having performance issues between revit+enscape, like mentioned above, it's almost certainly because your CPU isn't good enough or you don't have enough RAM.
mattendler - Reproduce the issue then please submit log files via the Enscape Feedback button.
Enscape doesn't support Multi GPU, so your log files will help us to see what the issue may be here.
Thanks in advance.
mattendler - No, that is not possible, as you can only assign the CAD to the GPU, not Enscape, as it will not be seen in the GPU control panel as it is acting as a 'slave' to the CAD, so to speak.
Once again, log files will help understand why you have this sluggishness when using Enscape in conjunction with Revit.