I've gotten 13 achievements to unlock with multiple log ins (it has worked 6 different times, so far). This is how I did it...
Step 1. Create a new world with a dummy account
Step 2. Press start on controller 2, and THEN sign in with your main account (do NOT sign in, before you join the game).
Step 3. Sign out with your main account.
Step 4. Repeat steps 2 & 3.
Just leave the dummy account idle with controller 1. You do not have to keep backing out, and resigning in with your dummy account, nor do you have to use multiple dummy accounts. One is enough. It won't work every time, but I promise it works.
I keep editing my post, because it keeps working! I'm even unlocking ones, that nobody has mentioned, so I'm thinking they can probably ALL be obtained with this method.
Passing the Time
Sniper Duel
Into Fire
Return to Sender
The End
Adventuring Time
The Haggler
Stayin Frosty
Pot Planter
Have a Shearful Day
Rainbow Collection
Renewable Energy
Zombie Doctor