Combining/Merging two Level 1 RAIDs (or their filesystems)

Hello,
I have the following setup:

on both of these raid1s I have an ext4 filesystem (one is allmost full, the other empty).
I want to combine them, so I can use the space on them simultaniously so it looks similar to this from the outside:

I have read, that mergerfs could do something like this but I have no Idea how to use it. I could not find any introduction tutorials for mergerfs and omv. I have backups in place in case something goes wrong, but I would like to keep my data while combining the two raids.

If I missed any information, please tell me, I just want a 7TB filesystem (with redundacy) that I can share on my network.

Thank you for your help in advance!

Thank you very much! I'll try the plugin and post how I did it here. For everybody else being a noob and having trouble finding the plugin, this link might be helpful: http://omv-extras.org/joomla/index.php/guides

Okay, it was really easy. I downloaded the unionfilesystem plugin after installing ovmextras. Then I went to the union-filesystems tab, added a new one combining the two. Apply. The only thing left to do was changing the drive of the shared folder and apply the change again.