RPi4 kodi grab hdmi signal from hdmi1

[unformatted]

I'm running kodi standalone on rpi4 (aarch64, linux-rpi) and try to capture the hdmi signal from the hdmi1 port. On hdmi0 I have a monitor connected and on hdmi1 an hdmi video grabber which usb output is connected to my desktop rpi4. I use QT V4L2 test Utility on my LXDE desktop to show the output of the grabber.

When I boot the kodi pc, on my desktop pc the grabber shows the boot sequence but after the target graphical interface is reached, it stays black (no kodi GUI) while the monitor on the kodi pc shows the kodi GUI.

How can I grab the kodi GUI from the hdmi1 port?

Here's the kodi pc configuration:

[user@PC19 ~]$ cat /boot/cmdline.txt
root=/dev/mmcblk0p2 rw rootwait console=serial0,115200 console=tty1 selinux=0 plymouth.enable=0 smsc95xx.turbo_mode=N dwc_otg.lpm_enable=0 kgdboc=serial0,115200 audit=0

[user@PC19 ~]$ cat /boot/config.txt
initramfs initramfs-linux.img followkernel
dtoverlay=vc4-kms-v3d,cma-512
dtoverlay=
dtparam=audio=on
dtparam=spi=on
dtoverlay=disable-wifi
dtoverlay=disable-bt