multi gpu also for viewport? does having more than one gpu also have an impact on speed of the rendered viewport in cycles ? (renderd viewport i mean the traditional shift z render )

or multi gpu rendering is only okay for the f12 final render ? 

and about eevee ? doest is support multi gpu for viewport as well as f12 final render ? 

thanks, im planning to upgrade my pc and i want to be sure before i make the purchase
Yes, multiple GPU's will also be used for the shift+z viewport render.

No, multiple GPU support is not planned for Eevee yet.
https://devtalk.blender.org/t/is-will-eevee-support-multi-gpu/625
It will just use your "display" card.