It is possible to use both a GPU (Graphics Processing Unit) AND onboard video together? - Yes.
When you install a graphics card, the integrated GPU is automatically disabled to avoid resource conflicts.
In this case, if your CPU has 2 outputs and your GPU has 2 outputs, you could use all 4 outups.
However, the BIOS on some motherboards does support simultaneous use of both GPUs, so you''ll have to check what graphics options are available in the BIOS configuration. Be aware that this is only supported on some motherboards.
If that option is not supported then you must connect monitor(s) to your graphics card.
Thx to @Carlos M. & @Robert H for their help in this topic.