Getting a new graphics card. Should I leave my old graphics card in there too?

After discussion with Adobe tech support it seems I may be getting crashes because my current card has too little memory so I've ordered a new one.

My old card is  GTX 760 with 2GB ram and the one I've ordered is a GTX 1080 with 8GB.

When I install the new card, should I leave the old on in? Assuming it fits and there's power of course.

I've read Premiere has support for multiple cards:

https://www.provideocoalition.com/adobe-premiere-pro-and-multiple-gpus

but does it make sense in my case?

Where should I plug the monitors?

All in the newer card, or split between the two?

Thanks for any help.