This seems to imply that “graphics display offloading” only works between the nvidia and modesetting Xorg video drivers, not even between two devices using the nvidia Xorg video driver. Your experience seems to confirm this, but I’d ask on the NVIDIA forum, maybe the developers can confirm/confute it. If this is indeed the case, then there may be something in NVIDIA Xorg driver to accomplish this task (maybe SLI or similar?).
If there’s not, then the nvidia kernel driver has an option NVreg_GpuBlacklist, which could potentially enable you to use the nouvau kernel driver for the GTX 750 Ti, and you might just be able to configure the modesetting Xorg video driver for that device, and if the stars align properly, there’s a slim chance that the “graphics display offloading” feature of the nvidia Xorg driver will work. Although I’m not completely sure if the modesetting Xorg driver can be used with nouveau-driver GPUs.
There’s certainly room for experimentation, but I’d first ask around on the NVIDIA forum, to see if nvidia-nvidia “graphics display offloading” is possible similarly to nvidia-modesetting.