I recreated this and it always needs only 1 C4 on both sides. every time ;)
I can see in your video where you put the second c4, the right wall took damage too, so it has been absorbed from the left wall ;) Take a look, after the c4 on the left wall, the right wall was going black ;)
This is also true, when I did my testing I found that attached walls absorb some of the damage. So when I did my c4 test I also made sure there wasn't any walls near by. You can also see this when you repair the corresponding walls that they have absorbed some damage.
All-in-All it does seem a little random and I still place metal doors on any wall on the main floor,2nd floor, or even 3rd floor if I think people can easily get to it.
I understand the idea that other objects "Absorb" damage but the damage is definitely not split evenly between the two walls. The other wall is simply taking damage because it too is in the explosion radius.
Notice that the second wall was standing on both my explosive tests, not just one of them.
You can test this for yourself, a metal door takes 11 grenades to destroy completely. Theoretically it would take 22-24 grenades to destroy two metal doors standing side by side, however you can destroy two metal doors on a corner with 14 grenades placed perfectly in the middle of them, or three metal doors with 18 grenades. Or even four metal doors one on each wall surrounding you with 23-25 grenades.
This is not them "Absorbing" damage, as four metal doors would be 44 grenades. This is simply them taking less damage because the area you must place the grenade to hit all the doors is farther away than just destroying one door.