Mounted directories inside lowerdirs not available in merged directory of overlayfs

Question

Lets say I have two squashfs-images, containing parts of a filesystem.

└─> tree one-fs/
one-fs/
└── usr
    ├── firmware
    │   └── onefirmware
    └── lib
        └── onelib



└─> tree two-fs/
two-fs/
├── twosomething
│   └── somethingtest
└── twotest

I mount the first squashfs-image (one.sqfs) to a folder called one. Then I mount the second squashfs-image (two.sqfs) to a folder called two/usr.

mkdir one
mkdir -p two/usr
mkdir three

sudo mount -t squashfs one.sqfs one
sudo mount -t squashfs two.sqfs two/usr

After doing so, I use overlayfs to merge both folders, like so:

sudo mount -t overlay -o lowerdir=one:two overlay three/

So I dont use the mountpoint two/usr but the folder two as lowerdir. Unfortunately, in the merged folder three, there is no folder usr with the contents of two.sqfs available.

└─> tree three/
three/
└── usr
    ├── firmware
    │   └── onefirmware
    └── lib
        └── onelib

Can someone enlighten me why it seems, that mounted fs inside of the lower directory are not merged and treated as empty directories? Also swapping one and two in the overlay mount command does not change anything.