> Is there a way to only enable HDR on the HDR capable monitor.

It is indeed possible.

> You might need to adjust the relative brightness of SDR and HDR content for your HDR-capable display. This is because a display interprets an HDR and SDR signal differently, and the results will vary by make and model.
> 
> 1. Select the **Start** button, then select **Settings > System > Display**.
> 2. If you have multiple displays connected to your PC, select the HDR-capable display under **Select and rearrange displays**, and then
> move the Settings app window to the HDR display that you’re adjusting
> 3. On the **Display** settings screen, select **Windows HD Color** settings.
> 4. Under **SDR content appearance**, drag the slider to get the right balance for brightness between HDR and SDR content.

Source: [HDR and WCG color settings in Windows 10][1]

> Problem: All standard dynamic range (SDR) content and apps appear too bright or too dark on an HDR-capable display.

> Solution: Since the brightness of SDR content is already set, the SDR content appearance setting will change the brightness of HDR content relative to the brightness of SDR content.

Setting the SDR brightness level on all your monitors should solve your problem.


  [1]: https://support.microsoft.com/en-us/help/4040263/windows-10-hdr-advanced-color-settings