My new monitor only has DP and HDMI inputs. both my nvidia gpus have only 2 DVI outputs. My motherboard has a HDMI tough. Is it possible to set the system up in a way that I use the HDMI output of my motherboard, but the two nvidia GPUs act as accelerators or something like that?
You are thinking of the VooDoo accelerator cards that where popular 15 years ago, aren't you ?
No. That won't work.
The HDMI on the motherboard is strictly intended to be used for the build-in graphics provided by the CPU.
You can't use the GPUs on the Nvidia cards as "add on" accelerators. They deliver their own output on their own DVI ports.
DVI to DP and DVI to HDMI converters are cheap. Get some. That solves your problem.