Install it over 0.8.9. (This is the same file as the 0.8.10-rc1 test build, so seeing "rc1" in your log is normal.) A small fix: enemies a friend's game spawned could not be hurt by you.
It still uses protocol 35, so it plays with 0.8.9, but the fix works only in the games that have it: everyone should update.
What changed
Your hits on an enemy that a friend's game spawned (random encounters, ambushes, and the like) now stick, and a friend's shot no longer heals what you took off it. Before, the enemy's health went straight back up for the player whose game didn't spawn it, so only a hit that killed it outright counted, and a hit from the other player could heal it in its own game (the Deathclaw that kept regenerating).
Known issues
If both of you hit the same enemy within the same instant, one of the two hits can still be lost.
The rest are as in 0.8.9: friends' faces untried with LooksMenu, the stand-in's grenade not shown in its hand during the wind-up, Dogmeat squatting on the joiner, a friend's parked frame taking a moment to move when first synced.