Summary:
The double sea grass hitbox is actually separate. There is a small gap between the two halves of the hitboxes. This is inconsistent with other twin plants that have no gaps between the hitboxes.

Steps to reproduce:

1. Place sea grass.
2. Use bone meal to turn sea grass into double sea grass.
3. Hover in the spot between the top and bottom half of the double sea grass hitboxes.
4. Try to break the double sea grass.

Observed results:
Double sea grass hitbox separate (separate upper and lower parts).

Expected results:
The hitbox will be intact, just like similar double plants.

Image: