Ups, i think i have missunderstand you

Maybe you can use the commandpromt and start Octane with that.
The syntax for the GPU´s is -g 0 -g 1 -g 2...
"-g 0 -g 1" select device 0&1
"-g 0 -g 1 -g 2" select device 0, 1, 2
To test, you can use the commandline
-n "xxxxx" = new project and the filename
-m "xxxx" = meshname in Octane
-l "xxxxx" = Link the meshname with this file
Octane.exe -n "G:\Octane\Project\test.ocs" -m test.obj -l "G:\Octane\Project\test.obj" -g 0 -g 1 -g 2
face