You should be able to render using two or more Octane instances in the same machine with only one license. The GPU settings are set for the current rendering, so you could for example open one Houdini session and render from it using one GPU, and later open another Houdini, select other GPU and also render from it without problems.
You can even configure the GPUs to be used for the current rendering from hython or a script using the "Octane_setGPU" hscript command.
Hope it helps,
Computer Graphics Software Developer
Octane plugin development and support: LightWave and Houdini